The FlightGear flight simulator project is an open-source, multi-platform, cooperative flight simulator development project.
0 A.D. (pronounced „zero ey-dee“) is a free, open-source, cross-platform real-time strategy (RTS) game of ancient warfare. In short, it is a historically-based war/economy game that allows players to relive or rewrite the history of Western civilizations, focusing on the years between 500 B.C. and 500 A.D.
Arena Live is a free, open source, Quake-based massive multiplayer, first person shooter.
Nexuiz is a free open-source first person shooter that runs on Windows, Linux and OSX.
Red Eclipse is a Free and Open Source first-person shooter that runs on Windows, Linux and OSX.