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.
Xonotic has incredibly good graphics for an opensource game, while the gameplay is of course slightly inspired by several other games, but it is essentially a unique mixture of mostly original and different ideas which really make the game stand out. It could be considered most similar to Unreal Tournament and Quake at its heart, especially regarding teamplay and game mechanics.