Besides the earlier posted list, I also found some other stuff we could play together under F2P. I don't have any of these games, but I think it might be cool to play these.
APB-This was a disaster when it first launched, but another company took it over and fixed it. Think GTA mixed with an MMO.
Rusty Hearts-A pretty cool lookin', cel-shaded, anime-style, hack 'n slash MMO. Lot's of dungeon crawling and there's even PvP modes!
Poxnora-a strategy game that looks to be a mix of Pokemon and grid-based games like FF Tactics or Disgaea.
There's also Champions Online and DC Universe Online for anyone who wants superhero stuff. :3
If nothing else, I'd really like to play Spiral Knights. That game looks great. Pretty cool that I can play it on my own too. Good thing I have a gamepad for my PC.