Cube 2: Sauerbraten!

Xonotic is a free (GPL), fast-paced first-person shooter that works on Windows, OS X and Linux. The project is geared towards providing addictive arena shooter gameplay which is all spawned and driven by the community itself. Xonotic is a direct successor of Nexuiz with years of development between them, and it aims to become the best possible open-source FPS (first-person-shooter) of its kind.

Red Eclipse is a fun-filled new take on the first person arena shooter, built as a total conversion of Cube Engine 2, which lends itself toward a balanced gameplay, with a general theme of agility in a variety of environments.

Overwatch is a team-based shooter where heroes do battle in a world of conflict.

Warsow is a completely free fast-paced first-person shooter (FPS) for Windows, Linux and Mac OS X, set in a futuristic cartoonish world where rocketlauncher-wielding pigs and lasergun-carrying cyberpunks roam the streets.

Unvanquished is a free, open-source first-person shooter combining real-time strategy elements with a futuristic, sci-fi setting. It is available for Windows, Linux, and Mac OS X.

OpenArena is a community-produced deathmatch FPS based on GPL idTech3 technology. There are many game types supported including Free For All, Capture The Flag, Domination, Overload, Harvester, and more. There are 17 unique player models to choose from and 12 weapons.

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.