Nexuiz: An Awesome FOSS Game for Linux (Oh Yeah, Windows and MacOSX Too)

Nexuiz is a free open-source first person shooter that runs on Windows, Linux and OSX. It's a 3d deathmatch game that has been in development since the summer of 2002. The goal of the project is to create a high quality first person shooter that can be played freely across all platforms in one package: PC, Mac, and can be even included in Linux distributions. The game is entirely open source, with the sources of the engine, gamecode, map files, and compiling tools, all included with the download, all under the GPL license.