DoW is a goodun for sure, if a little older
but Supreme Commander is the ultimate online RTS IMHO.
NWN2 is good in multiplayer, and more player adventures are coming all the time. The second expansion is on its way and should be even better for multiplayer.
NWN1 doesn't look so pretty, but is very mature and there are a huge number of both adventures and persistant worlds to play multiplayer.
Two Worlds apparently has an okay multiplayer - I couldn't stand the single player but I have friends who like it.
Team Fortress 2 and Counter Strike:Source are well established online FPSes
edit: oi! Stop reading my mind Salazaar!
edit2: Dungeon Siege 2 might be another one to look out for.