Xonotic

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 the Nexuiz project with years of development between them, and it aims to become the best possible open-source FPS (first-person-shooter) of its kind.