With the economy down, free games are always a good thing. But what MMO's are your favorite?
A few that I like are...

Anarchy Online: one of the better sci-fi MMO's out there, lots of interesting classes, best skill customization I've seen in a free MMO, a developed world and lots to do.
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Dungeon Runners: At first, seems like a cheap imitation of WOW, but then you notice the little things like the jokey dialog and the amount of skill customization with the character.

Dream of Mirror Online or DOMO: I love the cell-shaded graphics. There's a bunch of jobs you can choose from and although I've just started playing it, the game seems like it has a lot you can do.

Shin Megami Tensei: Imagine : I'm actually in the process of downloading this and if anyone has played it, go ahead and elaborate. It looks cool, kinda modern with mixes of Pokemon and JRPG elements along with a lot of social fun-ness, but I guess I'll find out soon :D
I've played others, just those few I mentioned are the ones I've found unique and high in quality.