Let me complete\fix LRK's guide:

DM - Dark Mercenaries
Active members: FireSworD, DizZy, MaGoo, AvK, Spotty, EvilMike, FQuist
All members: FireSworD, DizZy, MaGoo, AvK, Spotty, EvilMike, FQuist, iCeD, WhiteFang, Syntax, Sprintman, VelKasha, others
Overall skill: Very high
Player activeness: Very high
Clan activeness: Very low
New member strictness: Very high
Has some of the best players on JJ2, it's players play a lot, but the clan itself doesn't do much in terms of wars and getting new players.

FoR - Fellowship of the Ring
Active members: DarkShadow, TurtleSlayer (Turtle), JmaN, CelL, Boarder
All members: DarkShadow, TurtleSlayer (Turtle), JmaN, CelL, Boarder, FireBall, RagE, some others
Overall skill: High \ very high
Player activeness: Low
Clan activeness: Very low
New member strictness: Medium / high
Has lots of great players, but not very active. Only one of the members plays regularly.

[Si] - Semi-idle
Active members: BlurredD (Blur), Scizor CT (Sciz CT), Blackraptor (Blacky), Satan
All members: BlurredD, Scizor CT, Blackraptor, Satan, Jackazz, URJazz, Darthmaul
Overall skill: High / very high
Player activeness: High
Clan activeness: Very low
New member strictness: Very high
A great clan with a limited number of members. I don't reccomend trying to join, though. It's hard to get in.

TF - Total Freaks
Active members: Unhit, Enigma, Dalspots, LAbRaTkiD, Thunder, Stiletto
All members: Unhit, Enigma, Dalspots, LAbRaTkiD, Thunder, Stiletto, Skulg, WhiteRabbit, Dopeh, Flash, some others
Overall skill: High
Player activeness: Medium / high
Clan activeness: Medium
New member strictness: Medium
Has lots of great players, fairly active, and does clanwars. Also plays JediKnight 2, JediKnight 3, and Warcraft 3, although only active in WC3 and JJ2.

GpW - Gurus of Proactive Warfare
Active members: ChavRash, SuperJazz, Gummi, Newspaz, Pyromanus
All members: ChavRash, SuperJazz, Gummi, NewSpaz, Pyromanus, Shadow, Ax Man (Axie), Disguise, Electric, Beauman, Spaztic, Veggieman, others
Overall skill: High
Player activeness: Medium
Clan activeness: Very low
New member strictness: Medium / low
Has lots of great players, but some of the best ones rarely play.

MT - Monkey-E Team (I remember it being called Mighty Turtles at one point though)
Active members: R3ptile =D
All members: R3ptile, Frudo, Kakashi... Lalalala
Overall skill: OMG 1337!!
Player activeness: Medium
Clan activeness: Medium / high
New member strictness: High
Has some good players, mostly from Israel. Does clanwars. Periodically dies and comes back.

CC - Cracco Clan
Active members: CraccoBoy, Anakin, Drunkbunny, Vegito, Scare, Tublear
All members: Too many to list (too lazy, rather)
Overall skill: High
Player activeness: High
Clan activeness: Medium / low
New member strictness: Medium / low
Probably the biggest clan on JJ2.

RF - Rabbits of the Future As known as XSD
Active members: Yasco, Squirrel, Snooze, Jippo, Drunkbunny
All members: Yasco, Squirrel, Snooze, Jippo, Drunkbunny, Asmodeus, DarkSonic
Overall skill: Medium / High
Player activeness: Very high
Clan activeness: High / very high
New member strictness: Medium
Just XSD under a new clantag.

FE - Final Existence
Active members: Optic (Ninja), Nimrod
All members: Optic, Nimrod, Xtreem
Overall skill: Medium
Player activeness: Medium
Clan activeness: Very low
New member strictness: Very low
This clan isn't really a JJ2 clan. It's mostly a UT clan. If you want to get in, Nimrod will probably let you.