As I've promised, here is an official list of all available Len in MUGEN that have been released, or at least that we've seen here. I may be missing more, so feel free to post them below so I update this list. I put name of character and creation, website to download it, last update so you know if you have to upgrade the character (all of them has been tested with last update), a description of her gameplay, a screenshot showing how she looks like and my opinion about the character. Last three are in Spoilers.
Also, have in mind that some are from the niconico uploader (that site where you can find a lot of japanese releases with names such as up2505, for example). For those ones, I'm not sure if I really got the latest version. And it's possible that in the future they become unavailable. If you know they've been posted in an official site or you found a most recent version that the one I posted, tell me so I update it (also tell me when it turns out unavailable).
Btw, date works as DD/MM/YYYY
Name: Alt
Creator: hanayakuro
Website: http://hanayakuroalt.blog134.fc2.com/ (Press "Continue Reading" to find the link, it's the 1.0+ version)
Last Update: 09/04/2011
Spoiler, click to toggle visibilty
Description: A Len with two modes, one uses bullets and fire and the other lasers and energy balls, that you can change ingame by just pressing "z". She heavily relies on the bar that you can see in the bottom-left of the screenshot below to use her specials and supers (supers spend power too). She's slow too.
Screenshot:

Opinion: It's not bad. It relies too much on that bar that fills too slow. IMO we wouldn't miss it if it was removed. She's also kind of slow at moving, which doesn't really fit her moves, that are fast. Still, it's worth downloading to see if it fits you.
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Name: Annu
Creator: Rien
Website: http://cid-618f41093282183c.office.live.com/browse.aspx/%E3%82%AD%E3%83%A3%E3%83%A9
Last Update: 30/05/2011
Spoiler, click to toggle visibilty
Description: Len that impersonates Fritz. She has his moves plus some extra. Follows his gameplay in the source game.
Screenshot:

Opinion: Well, it's okay, but she's faster than Fritz and even has a long distance attack. Apart from that, it's not bad. Still, you will probably wish the creator would add more originality here.
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Name: Astranalent
Creator: khn
Website: http://www.mediafire.com/?sharekey=6ea8b1d67e61f2d40c814df2efeadc50ed6a9ab138d973863e34c0a955f98962
Last Update: 03/06/2011
Spoiler, click to toggle visibilty
Description: A Len that uses guns and scythe. Custom gameplay from khn. She is a reference to Dis Astranagant from Super Robot Wars, btw.
Screenshot:

Opinion: khn always surprises me. The character is amazing. She's really original and interesting. Her moves are flashy and balanced, a combination I love. Some may do a little too much damage, but overall it's an awesome release. Go get it.
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Name: Dim
Creator: M2pa
Website: http://muvluv-alternative.game-server.cc/cgi-bin/mugen-up/upload.cgi?mode=dl&file=938 (Pass: Dim)
Last Update: 15/01/2011
Spoiler, click to toggle visibilty
Description: A really slow and really heavy Len. When she walks the screen shakes, and when she jumps she falls realy fast to the ground, like if she was really heavy (and can do damage if she lands near a downed opponent). She has three bars below, one is health, the other makes the 3rd bar fill faster and the last, when fully filled, makes her go into EX mode for all the round. Her moves are all for close distance, except when she enters the EX mode. Some give extra effects for some time, like more defense or poison.
Screenshot:

Opinion: Not good. She's really slow, yet all her moves are close distance, which she's terrible at because of her speed and small range. If she had superarmor for one hit, maybe it would make a difference. The EX mode is cheap and makes her do a lot of damage, her attacks cover quite a lot, and all effects last all the round, but takes so much time to fill that is not worth it. Try it if you want, but personally, you can skip her.
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Name: Fiorenza
Creator: GAB
Website: http://cid-cbe7b10d2ff1aa7a.office.live.com/browse.aspx/.Public
Last Update: 14/12/2010
Spoiler, click to toggle visibilty
Description: A Len with GG gameplay. Has some moves from Dizzy and Justice.
Screenshot:

Opinion: It's not bad, but it's not good. It's really hard to control her movement, since you easily trigger superjumps all the time, making aerial combos quite imposible to do. The attacks that makes her summon a Neko Ark do too much damage. It's decent at least.
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Name: Gear Len
Creator: GAB
Website: http://cid-cbe7b10d2ff1aa7a.office.live.com/browse.aspx/.Public
Last Update: 14/12/2010
Spoiler, click to toggle visibilty
Description: A Len with GG, CvS2, MB, Touhou and more modes selectable before the fight starts.
Screenshot:

Opinion: It's just a horrible mix of different gameplays in one single character, which turned horrible since none of them have been carefully done. Skip it.
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Name: Godzillen
Creator: k
Website: http://muvluv-alternative.game-server.cc/cgi-bin/mugen-up/upload.cgi?mode=dl&file=2662 (Pass: kkk_2020)
Last Update: 11/07/2011
Spoiler, click to toggle visibilty
Description: Godzilla, aka throws fire, uses her tail to attack, pounds the ground and can grab and throw the opponent easily.
Screenshot:

Opinion: It's good. Her projectiles do too much chip and normal damage, though. Apart from that, she's fine. Download it.
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Name: HAKU=LEN
Creator: ShowBuySpirit
Website: http://aifee.syuriken.jp/
Last Update: 08/06/2011
Spoiler, click to toggle visibilty
Description: A Hakumen conversion to MUGEN. Has all of his moves except 236A. It also uses BB gameplay (such as Guard Barrier, for example).
Screenshot:

Opinion: It's really good. It's the best Hakumen you'll find right now, and it's similar to source. If you like Hakumen, but hate Len, you will probably have to do a decision here. The other Hakumen released in MUGEN use his sprites from the game, but they're not really accurate and their quality is questionable. It's your choice in the end.
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Name: Hime Len
Creator: M3
Website: http://page.freett.com/mugen_m3/
Last Update: 17/04/2011
Spoiler, click to toggle visibilty
Description: A Rachel conversion to MUGEN. Has all of her moves. It also follows BB gameplay, though some things are missing.
Screenshot:

Opinion: It's really good. Best and only (not sure) Rachel in MUGEN. Again, if you hate Len but like Rachel, you'll have to choose. Personally, M3 is a great creator, so regardless of your hatred to Len, consider downloading it.
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Name: KaLen
Creator: BluishCat
Website: http://cid-f8a3c3474cb9a76e.office.live.com/browse.aspx/.Public/MUGEN%E9%96%A2%E4%BF%82/%E3%82%AB%E3%83%AC%E3%83%B3
Last Update: 08/06/2011
Spoiler, click to toggle visibilty
Description: It's a joke character. She uses moves from other characters, can throw even E.Honda as a projectile, etc. Also, she uses HnK star bar (the bar that tells you if you can use the Instant Kill or not) and another bar at the bottom left, that allows you to use different moves when it's filled.
Screenshot:

Opinion: It's really funny. Seeing her throw Honda or even Bison as projectiles is hilarious. She's also well done, so you can use her as a normal character. Yeah, I really recommend you to download it. And use her Instant Kill. It's Sumou Power, that's the only thing I can tell you.
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Name: KouyaLen
Creator: Zenjidoh
Website: https://skydrive.live.com/?cid=C4E6793C66ADEDBE
Last Update: 10/07/2011
Spoiler, click to toggle visibilty
Description: She uses baseball moves. She can hit balls thrown at her and depending if you hit earlier or not, the ball goes in a different way. She can also grab and use the opponent as a ball. And Heavy D appears as the one who throws her the ball and other things.
Screenshot:

Opinion: Well, she's okay. Nothing great, though. After seeing her new updates, I'm not really expecting it to be as great as I expected. It's interesting, but it's probably not going to be in your roster.
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Name: Len & Len
Creator: Khn
Website: http://www.mediafire.com/?sharekey=6ea8b1d67e61f2d40c814df2efeadc50ed6a9ab138d973863e34c0a955f98962
Last Update: 20/02/2011
Spoiler, click to toggle visibilty
Description: Both Len and White Len in one character. You can switch from one to another by pressing Start. There's also attacks that allows you to use the other Len that is not controlled at the moment. Both of them have MAX mode, though they reach it in a different way.
Screenshot:

Opinion: It's a really good character. The only thing I miss is a super that involves the two of them in the same attack (not just one doing an attack and the other another). But apart from that, the gameplay is really good and I see nothing wrong in it. Get it.
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Attention, read this first!
Spoiler, click to toggle visibilty
Lenja seems to have a problem when it's used with MUGEN 1.0, to solve this do these steps:
- Go to the character's def and change mugenversion to 1.0.
- Go to the character's cns and search for "var(2)=%f, var(1)=%D, var(32)=%d, movehit=%d,Y=%d \n", then change var(1)=%D for var(1)=%d.
Problem solved. Thanks to fhqwhgads7 for his help.
Name: Lenja
Creator: 3ha
Website: http://page.freett.com/3ha/
Last Update: 16/08/2010
Spoiler, click to toggle visibilty
Description: Suija from SS transformed into a Len. Haven't used him in the source game, but according to people it seems to be similar to Suija from source.
Screenshot:

Opinion: Awesome. I've never used him in source, but I still think this character is awesome. Gameplay is good and I don't find anything worth mentioning, except for its incompatibility with MUGEN 1.0 that you can solve above and that every attack makes a red screen appear for a second, which is kind of annoying sometimes. Download it, guys.
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Name: Lian
Creator: Marktwo
Website: http://BLACKLISTED WEBSITE.ninja-web.net/
Last Update: 18/02/2011
Spoiler, click to toggle visibilty
Description: Basically, a Len that uses Wind moves. Can't really say a lot more than that. She also uses some Geese Howard moves too, like Reppuken.
Screenshot:

Opinion: It's good. Only problem is that she has some infinites that work if your character doesn't have air recovery. Overall, it's worth downloading.
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Name: Luu
Creator: nadagetsu
Website: http://muvluv-alternative.game-server.cc/cgi-bin/mugen-up/src/up2424.zip.html
Last Update: 19/06/2011
Spoiler, click to toggle visibilty
Description: Len as a wolf. Uses her fangs and claws to attack, among other things. Nothing more to say.
Screenshot:

Opinion: Incomplete, that's how it feels like. I personally think she could have more than just two normal attacks. The animation of her attacks feel wrong, and the grab causes problems. One super clashes with her style too. It's meh.
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Name: N-Len
Creator: Ouchi
Website: http://mugezon.web.fc2.com/
Last Update: 04/06/2011
Spoiler, click to toggle visibilty
Description: A Len that has Nero Chaos and Neko Arc Chaos moves. She also has a BASARA Finish. Her gameplay is more focused on combos. Can also summon some helpers from different games/animes.
Screenshot:

Opinion: It's not bad. Damage output in some moves is a little high, and it also feels like they just put Nero and Neko Chaos moves together in a mixer, but overall it's worth trying and play to see how it feels like.
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Name: Pale
Creator: M2pa
Website: http://muvluv-alternative.game-server.cc/cgi-bin/mugen-up/upload.cgi?mode=dl&file=502 (Pass: pale)
Last Update: 20/11/2010
Spoiler, click to toggle visibilty
Description: She's slow, yet she has some projectiles, attacks that makes her teleport near the opponent and long range attacks that makes up for her speed. She can also teleport to move fast, but spends the bar below. The bar below is divided in two parts. The red is the one that allows you to teleport and do specials/supers. It fills really slow, unless you fill the blue bar, which accelerates the recovery speed of the red bar as long as it's filled.
Screenshot:

Opinion: Unlike Dim, this one is way better. The only problem is that it relies too much on the bar below for the majority of things, and personally, I could play without that limiter. I also don't understand why she has to move so slow, it's unnecesary. It's good, and worth trying.
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Name: Ren
Creator: Lycoris
Website: http://redandwhite.blog67.fc2.com/
Last Update: 23/07/2010
Spoiler, click to toggle visibilty
Description: A Len that uses fire moves. Can throw explosive cards too, that can act as mines if thrown to the ground. Can also throw Neko Arcs as missiles. She also has a move similar to Joker from Oswald.
Screenshot:

Opinion: It's a good Len. She has interesting moves and there's nothing cheap on her (maybe the super that makes a lot of Neko Arc fall from the sky). Download it, you will like it.
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Name: ROBO-LEN
Creator: Ukege
Website: http://sokowodokunda.ma-jide.com/html/index.html
Last Update: 04/06/11
Spoiler, click to toggle visibilty
Description: A Robot character, similar to Ky and other Robot characters made by Ukege. Uses Megaman moves. She also has a curse like effect that is activated when three different cakes hit the opponent. The same happens to her if the bar at the top left is depleted after using too many specials/supers without replenishing it.
Screenshot:

Opinion: It's a good character, like other Ukege's creations. She also seems to respond to the majority of Len clones created so far. The only problem is that the move that gives you Megaman powerblocks can also give you items to refill your life, and there's even one that resurrects you again if you lose all your life. It's still good, though. Try it.
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Name: Sefu Len
Creator: Unknown (no name appears, only Electbyte, which is not the real name)
Website: http://muvluv-alternative.game-server.cc/cgi-bin/mugen-up/src/up0539.zip.html
Last Update: 23/11/2010
Spoiler, click to toggle visibilty
Description: A Len in a swimsuit. Nothing more to say. Moves are more for fanservice.
Screenshot:

Opinion: It's bad. I don't know if I have an outdated version or not, but it's bad. She's pure fanservice only, and one move even shows her naked for a moment. Her "b" attack animation is terrible, and she lacks proper effects and sounds. Not worth it.
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Name: S-Len
Creator: pprn
Website: http://cid-e3545520e659ce44.office.live.com/browse.aspx/.Public
Last Update: 04/05/2011
Spoiler, click to toggle visibilty
Description: A Len that uses Bison moves and also has Bison to help her. She can also summons ilusions of Bison.
Screenshot:

Opinion: Well, it's good, really, but since I've been using PotS' Bison, it's not really fair to compare PotS with this one. She's funny, that's for sure. There's one move that can be changed to Bison throwing her at the opponent, for example. You can even hit Bison in the taunt shown above, and he will fall, but the animation will still continue. It's worth downloading if you want a different and funny Bison, but if you want a good Bison just go for PotS.
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Name: Suilen
Creator: noeru
Website: http://muvluv-alternative.game-server.cc/cgi-bin/mugen-up/src/up2305.zip.html
Last Update: 09/06/2011
Spoiler, click to toggle visibilty
Description: Well, she's sorta like Warachia, but not the same. She can summon shadows of other MB characters just like him and uses claw attacks, but her moves are different. That's all I can say.
Screenshot:

Opinion: It's okay. Problem is, there's nothing that makes her great. She's slow for some reason, and some of her attacks are really difficult to use, even though they're easy to input. As I've said, it's decent, so you can try it, but there's nothing that makes you feel like "This character is great!".
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Name: Valentina
Creator: GAB
Website: http://cid-cbe7b10d2ff1aa7a.office.live.com/browse.aspx/.Public
Last Update: 24/05/2011
Spoiler, click to toggle visibilty
Description: A Len with, or at least tried to, GG gameplay. Can summon random characters from any games.
Screenshot:

Opinion: It's a piece of shit. It's really hard to control her since she just goes all over the screen when you try to move her a little. Damage from some attacks reach nearly 1-Hit KO. Her comboability is retarded. And Burst shouldn't be able to fill automatically and that fast. I don't know if GAB tried to do a joke character or not, but either way he failed horribly. Skip her.
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Name: Xehrnyth
Creator: LucifelShiningL
Website: http://mugenguild.com/forumx/index.php?topic=119553.0 (Posting a link to MFG because he's still uploading it and it's better you go to that thread and check it. Also give him feedback if you can, please)
Last Update: 06/06/2011 (Today)
Spoiler, click to toggle visibilty
Description: She's a glass cannon. Meaning, she's fast and some attacks do a lot of damage, but her defense is really low. Meaning, a normal combo can deplete her life really fast. She also has a cooldown bar that limits her.If that bar is fully filled, she will lose health really fast unless it's depleted.
Screenshot:

Opinion: Unfortunately, the cooldown bar depletes too fast to affect you at all. There's an option to disable the automatic recovery, but it should be outside the game, not inside, and it even doesn't work. That's all I will say here, since this isn't a place to post feedback. Download it and try it, you will like it. The creator is also in this forum, so go to his thread and help him, guys.
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Name: WLen_Force
Creator: M3
Website: http://page.freett.com/mugen_m3/
Last Update: 17/04/2011
Spoiler, click to toggle visibilty
Description: A White Len that uses ice, but with a complete different gameplay than in source, and with different attacks.
Screenshot:

Opinion: It's M3, of course it's going to be good. Well, to be more objective, her gameplay is good, there's not bad things you can spot and her attacks are flashy and balanced, which is awesome. Yeah, you should get it if you want a custom White Len that is good.
Second part of the list at the 3rd page.