1) Free Computer Game: FlightGear
FlightGear is a free general aviation flight simulator, an open-source (GPL), multi-platform, cooperative flight simulator development project. It includes an extensive and accurate world scenery database with over 20,000 real world airports included in the full scenery set. Its many features make it a fun game.
2) Free Computer Game: BZFlag
BZFlag is an OpenSource OpenGL 3D networked multiplayer "battlezone capture the flag" tank game for Linux, Irix, FreeBSD Solaris, Windows, and Mac OSX. As an OpenGL application it requires an implementation of OpenGL that works on your system. Mesa is known to work. It rocks on hardware 3D systems. Game server lists are supported to help you find people to play against.
3) Free Computer Game: Frozen Bubble
Frozen Bubble is an addictive game developed by MandrakeSoft where players shoot colorful bubbles across the gameboard. In 2-player mode the goal is to outlive your opponent, and in 1-player mode, which has 100 levels, the goal is to destroy all bubbles. It comes with three professional quality 20-channel musical tracks, 15 stereo sound effects, and seven unique graphical transitions.