-Quake Live
-Wolfenstein: Enemy Territory
-Sauerbraten - I LOVE this game, simple gameplay, fast-paced. Interactive map-making.
-F.E.A.R: Combat - The complete multi-player portion of the original F.E.A.R. you just have to download and request a KEY. Allows you to play with people who own the retail game.
-Crossfire - I don't care for it but there is a large community that plays religiously.
-Warsow - very basic, beta stage I think, but good to check out because it's different/interesting.
-Open Arena - based on the Quake III Arena source code. It's pretty active. first release in 2005, latest build was feb 23 of this year.
And you can always play Tribes XD
Have a good one man, hope you find something you like.