F.E.A.R. Combat has been released recently, which is the free multiplayer variant of F.E.A.R, and is very enjoyable.
Furthermore, for MMORPG's I suggest you check out:
Eternal Lands - very small client (26 MB), but a very advanced 3d game.
MU Online - bit of diablo-like mmorpg, hack'n'slash
For first person shooters:
Tremulous - a refreshing game. Aliens vs Humans multiplayer game with RTS elements. (based on Quake 3 engine so should work on your pc...)
Nexuiz - unreal tournament clone, but with exceptionally good graphics (works on older pc as well with all options off)
Legends - free Tribes 2 clone. Seems to be very buggy though.
@chainsoar
I'm in a team who creates a 'matchmaker' for populous, which enables you to sit in a lobby, host/join a game and chat with other populous players. There are alot of mappacks available that you can play as well. And leagues enable you to compete with other players. There are also some clans active. So if you look for someone to play...
see it here