Alliance of Valiant Arms
Typical Korean free FPS, but with better graphics (it uses Unreal Engine 3)
Download and register

Battlefield Heroes
Free cartoony 3D shooter.
Download and register

BMW M3 Challenge
Small racing game released by BMW.
Download

F.E.A.R. Combat
Free multiplayer version of F.E.A.R.
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Quake Live
Browser-based copy of Quake 3 Arena
Download and register

TrackMania Nations Forever
Fun and fast online racing game
Download and register

Urban Terror
Free multiplayer shooter based on Quake 3 engine.
Download

Wolfenstein: Enemy Territory
Probably the most famous free shooter.
Download


Some more free FPS games