Wolfenstein: Enemy Territory is a Multiplayer Online First Person Shooter which sports some of the best graphics for a freeware first person shooter.
Beautifully rendered first person shoot 'em up
Americas Army 2.0 is a completely free Special Forces game with training missions, network play and much more
A totally free online 3D multiplayer deathmatch game which some say is the best first person shooter ever.
Alien Swarm is a shmup played with an overhead view where the player can chose to play one out of four Marine classes; the Tech, the Medic, the Officer, and the Special Weapons.