OpenArena is an open-source content package for Quake III Arena licensed under the GPL, effectively creating a free stand-alone game. You do not need Quake III Arena to play this game. For windows, linux and macintosh.

Link to Official Site

OpenArena is a Free Content project. Its goal is to create new, original media while providing a stand-alone multiplayer game in the process.

The current supported gametypes are as follows:
– Free for All
– Team Deathmatch
– Tournament
– Capture the Flag
– One Flag Capture
– Overload
– Harvester
– Elimination
– CTF Elimination
– Last Man Standing
– Double Domination
– Domination

Instant Gib and All Rockets weapons modes are supported.
Simulated players, or “bots” are also available for your personal pulverizing pleasure.