Counter-Strike (Series)!

Cube 2: Sauerbraten is a free multiplayer/singleplayer first person shooter, built as a major redesign of the Cube FPS.

Urban Terror started as a realism based "total conversion" mod for Quake III Arena. Nowadays, Urban Terror is played using the free, stand-alone, ioquake3 engine modified, which doesn't require Quake III Arena, in addition to being an add-on to id Software's Quake III Arena.

A free-and-open-source first-person-shooter aimed at improving the classic game, AssaultCube!

Cross Fire is a free online first-person shooter featuring persistent rankings and five game modes – including the exclusive stealth-action Ghost Mode.

Take to the streets for intense close quarters combat, where a team's survival depends upon securing crucial strongholds and destroying enemy supply in this multiplayer and cooperative Source Engine based experience.

TrueCombat: Elite is a FREE modification for the Enemy Territory game (also free). It is a realistic modern day FPS featuring state of the art models and textures.

Alien Arena is a free and opensource first-person shooter game that combines a 1950s-era sci-fi atmosphere with gameplay similar to Quake (series) , Doom (Series) and Unreal Tournament (series) . Alien Arena focuses mainly on online multiplayer action, although it does contain single-player matches against bots.

Day of Defeat: Source features multiplayer, team-based gameplay set in the WWII European theatre of operations. Players choose a role and tackle goal-oriented missions based on historical operations.

Squad is a 50 vs 50 multiplayer first-person shooter that aims to capture combat realism through communication and teamplay, emphasizing both strong squad cohesion mechanics as well as larger scale coordination, tactics and planning. It features large open maps, vehicle-based combined arms gameplay, and player-constructed bases to create a heart-thumping, visceral gaming experience which mixes organized tactical multi-squad planning with split second decision-making in real-world scale firefights. »