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.

WWW: http://www.xonotic.org/