America's Army!

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.

Zandronum is a multiplayer oriented port, based off Skulltag, for Doom and Doom II by id Software.

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. »

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.

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