Shooter

AssaultCube
(formerly ActionCube) It's a Cube mod (total conversion), team oriented multiplayer first person shooter (Counter-strike like and more). The gameplay is similar to ActionQuake and provides a more "realistic" gameplay experience than Cube. (ZLIB license) [win32, linux and bsd (x86), macosx (ppc)]
http://assault.cubers.net/

Cube
Singleplayer (2 game modes, savegames) and multiplayer (12 game modes) first person shooter. Over 100 maps, possibility of in-game map editing (even multiplayer). Central server list. Demo recording. Uses OpenGL and SDL. (ZLIB license) [win32, linux and bsd (x86), macosx (ppc), pocketpc (xscale)]
http://www.cubeengine.com/

Cube 2: Sauerbraten
Singleplayer (2 game modes, savegames) and multiplayer (12 game modes) first person shooter, built as a major redesign of the Cube FPS. Possibility of in-game map editing (even multiplayer). Central server list. Demo recording. Uses OpenGL and SDL. (ZLIB license) [win32, linux and bsd (x86), macosx (ppc)]
http://sauerbraten.org/

Eclipse-game
Eclipse is a driving and FPS style shooting game with a campaign. Story takes place on surface of Moon. OpenGL graphics and original soundtrack. Single-player game only.
http://www.drzizka.adslink.cz/Eclipse.html

Nexuiz
Nexuiz is a first-person shooter which started as a Quake modification in the summer of 2001. [GPL source code and data] (Windows, Linux, OS X)
http://alientrap.org/nexuiz/index.php

OpenArena
An open-source content package for Quake III Arena (free stand-alone game). Contains bots. (GPL) [win32, linux and bsd (x86), macosx (ppc)]
http://openarena.ws/