OpenArena -- Free Quake 3 Arena (GPL)!

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OpenArena is a free/open source 3D computer game that belongs to the "first-person shooter" genre. OpenArena had its first public test release on August 19 2005, the day after the Quake 3 engine GPL source code release and also the last day of Quake Expo 2005.

OpenArena is developed mainly using free/open source - e.g. its game engine is id Software's GNU GPL Quake 3 engine. OpenArena is entirely free as in speech (both engine, gamecode and data) - pure Open Content

It is still in very early development. In addition, a "missionpack" addon is planned, to take advantage of the also GPL'd Quake III: Team Arena source code.

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Read More:
http://en.wikipedia.org/wiki/OpenArena

Downloads:

Windows:
openarena-060-win32-installer | mirror | Mirror 2 | Mirror 3

Linux:
openarena-060-linux-release | Mirror

MacOSX:
OpenArena 060 Mac
Note: Requires Linux tarball!

Screenshots:
http://mods.moddb.com/5727/openarena/image-gallery/

release 0.70 and 0.71 are out now. You need both since 0.71 is an update of 0.70
More info at source http://openarena.ws/