Counter Strike Source, if you don't already have it. This game is unreal.
There aren't really any MMO's out right now worth playing. Unless you want to sink hundreds of hours into a character, then play wow, if you can get to the end game the PvP is alright. I am waiting for Darkfall, a skill based game with full loot. Haven't had fun in a game since I played the last one with those rules
However it isn't scheduled to release until 2008, even that is a stretch, I predict maybe this time next year.