Xonotic

Please note: Xonotic is currently in beta at version 0.7, but the app's version number reports as 1.0 in the Finder.
Xonotic is a free and fast-paced first-person shooter for Linux, Mac, and Windows.

It combines addictive, arena-style gameplay with rapid movement and a wide array of weapons. It is available under the permissive GPLv2 license.
Xonotic is a direct successor of the Nexuiz project. It represents years of development from its humble beginnings as a Quake-1-engine modification, and now aims to be the best possible Open Source first-person shooter available. It could be considered most similar to Unreal Tournament and Quake at its heart, especially regarding teamplay and game mechanics.