Linux-gamers.net has a great comparison of free software shooters on Linux.
There have been many free software first-person shooters (FPS) projects over the years, from modded Doom and Quake engines to enhance the existing games (ezQuake, EGL, ZDoom), to free art packs such as OpenQuartz or OpenArena. In 2002, along came Cube, a single and multiplayer FPS based on its own engine, including artwork, maps, models and an ingame map editor. In the freeware (and Linux compatible!) world a little-known game called Legends, a Tribes-inspired game, appeared yet remained closed-source. Filling the FPS gap in the open-source world has usually been left up to commercial companies who release their games with Linux support (i.e. Doom3, Unreal Tournament 2004, Loki Software’s work) or freeware games produced by commercial studios(i.e. America’s Army, Wolfenstein: Enemy Territory) or simply running Windows games run via wine. In the last few years a few built-from-scratch community-based FPS projects, most built on the GPLed Quake engines, have popped up, among them are Tremulous, Alien Arena, Nexuiz, and War§ow. Some have kept their art assets under a closed license (War§ow), while others have also released their art under an OSS license (Nexuiz), I consider both categories free software since well, software refers to programs, code and procedures, not artwork. For this comparison, we’ll take a look at active, robust and community-developed free software shooters. Most released free software shooters are designed for multiplayer, a logical step for a game developed in an online community, however most also feature a bot-based single-player mode. While others have compared such games before, this feature seeks to be a little more thorough and go a step further, ranking the following seven games: Alien Arena, Nexuiz, OpenArena, Sauerbraten, Tremulous, War§ow, and World of Padman. In ranking these games, gameplay, design, innovation and presentation (in that order) will be held as primary criteria.