Star Wars Gaxies: An Empire Divided, waaaay back when.
Gave World of Warcraft a shot, not a fan.
Played Vanguard, Saga of Heroes for a little while, wasn't a huge fan.
Played Eve Online for a while, not for me.
Now I'm waiting for Star Trek online (like that'll ever happen) and Stargate Worlds. Stargate Worlds looks like it has great potential.
If you're interested in stargate, or a fan of MMO's in general, I'd recommend going to check it out at www.stargateworlds.com.
There's a forum, and news updates about it periodically on the site itself. You can even sign up to Beta test it, although I wouldn't recommend taking judgements of the game itself from the beta... beta's are always buggy.
I played EQ1, EQ2, DAOC in the past but I would never consider Second Life. Two kinds of people playing - the ones with the weird "having sex dressed as stuffed animals" fetish and the people who just won't shut up about how much money they are making on it and how you should get in now and get yours (sounds like recruiting for a pyramid scheme).
I also heard it was very laggy and crashes a lot.
I'm also waiting on stargate worlds but I think it may be another Duke Nukem Forever...