Quake Live should be good - when it comes out.
Call of Duty. All of them. (love 4)
easily the best free FPS now,
http://combatarms.nexoneu.com/Intro.aspx
there has just been a massive overhaul to weapons added in the game, give it a go its free !!!!![]()
here's another vote for:
Wolfenstein: Enemy Territory
Still the only game I play regularly. I feel like this game was a big inspiration for team fortress 2, in that the game play is very similar, and several TF2 maps bear a striking resemblance to Wolfenstein: Enemy Territory maps.