Games
Marathon: Aleph One
Aleph One is a game engine currently in development by the Open Source community. It is based on the source code of Marathon 2: Durandal, a game created by Bungie Software in 1995. Since Bungie released the source code, Aleph One has matured to include a variety of new features and improvements.
Website: http://source.bungie.org/link/
Zombies
A turn-based game where the only goal is to outlast and destroy the zombies before they do the same to you.
Website: http://codenautics.com/zombies/
Armagetron Advanced
Armagetron is a multiplayer game in 3d that attempts to emulate and expand on the lightcycle sequence from the movie Tron. It's an old school arcade game slung into the 21st century. Highlights include a customizable playing arena, HUD, unique graphics, and AI bots. For the more advanced player there are new game modes and a wide variety of physics settings to tweak as well.
Website: http://armagetronad.net/
BZFlag
BZFlag is a free online multiplayer cross-platform open source 3D tank battle game.
Website: http://bzflag.org/
Battle for Wesnoth
The Battle for Wesnoth is a free, turn-based tactical strategy game with a high fantasy theme, featuring both single-player, and online/hotseat multiplayer combat.
Website: http://www.wesnoth.org/
Frozen Bubble
Colorful 3D rendered penguin animations, 100 levels of 1p game, hours and hours of 2p game, nights and nights of 2p/3p/4p/5p game over LAN or Internet, a level-editor, 3 professional quality digital soundtracks, 15 stereo sound effects, 8 unique graphical transition effects, 8 unique logo eye-candies.
Website: http://www.frozen-bubble.org/
Abuse
Abuse is a run and gun video game developed by Crack dot Com, and published by Origin Systems/Electronic Arts. It was released in 1996, and runs on DOS and GNU/Linux operating systems. An improved port of the game was released for Mac OS by Bungie Studios and for the Acorn Archimedes by R-Comp Interactive. The game has been in the public domain since the late 1990s. Now the game has been ported to the iPhone/iPod Touch platform.
Website: http://en.wikipedia.org/wiki/Abuse_(computer_game)
Aleph One
Aleph One is an open-source descendent of Bungie's Marathon 2 first person 3D shooting game. Aleph One supports Marathon 2 and Marathon Infinity data files (available below). A conversion of Marathon 1 called M1A1 and many third party scenarios and net maps are also available.
Website: http://marathon.sourceforge.net/
Flightgear
FlightGear is an open-source flight simulator
Website: http://www.flightgear.org/
Freeciv
Freeciv is a Free and Open Source empire-building strategy game inspired by the history of human civilization. The game commences in prehistory and your mission is to lead your tribe from the stone age to the space age
Website: http://www.freeciv.org/