I thought I'd make a list of a few of the free MMOs I'm aware of. I know there are more but I can't think of any specifics at the moment, so I will add to the listing later.

Fly For Fun is a 3D cartoony MMO, it seems to be fairly successful. I don't know a whole lot about it but there's some general info on Wikipedia

Fury is a 3D entirely PvP-centric MMO that was released as a retail game, but due to some unpopular changes made to the game it lost alot of its fan-base. It was extremely successful during beta and its initial stages, but no longer has as much activity. It still has a great system and for a free game it is definitely worthwhile.

Maple Story one of the largest free to play MMOs, only 2D but it's popular for a reason I guess. I don't know a whole lot about it but there's some general info on Wikipedia

Rappelz 3D game, free to play, from a fairly popular developer of free MMOs. I don't know a whole lot about it but there's some general info on Wikipedia

Runescape 3D game, has a free-play option (also can subscribe for more features). I don't know a whole lot about it but there's some general info on Wikipedia

Shadow Bane is a 3D MMO, it's fantasy-esque like WoW and other games but has it's own feel to it. Great PvP combat system as well.

Silkroad Online 3D and free to play. It's built around trade on the Chinese Silk road, there's also monsters you can fight. Not bad overall.

Second life isn't a fully free MMO, but it's still a very good one. Though you should be careful playing it, as some people, attempt to disrupt it. I got a swarm of flying penises heading in my direction one time when I was using the game.