FlightGear project is a sophisticated flight simulator framework for use in research or academic environments, for the development and pursuit of other interesting flight simulation ideas, and as an end-user application. One of the best flight simulators available today under any platform.
TORCS, The Open Racing Car Simulator, is a car racing simulation which allows you to drive in races against opponents simulated by the computer. Dozens of tracks, cars, and race combinations makes this game hours upon hours of fun. You can create your own cars fairly easy and even develop your own computer-controlled driver or robot in C or C++.
Vegastrike is a 3D space combat, flight, and trading simulation. Currently in Beta development. Current features include split-screen play, trading, exploration, and plenty of good ol' science fiction shoot'em up action.
GL-117 is a small and fun 3D action flight simulator featuring 20 missions, different fighters, a random terrain generator, lighting effects, sound effects, music, and joystick support. An impressive flight simulation when taking into account its very small size.
SuperTuxKart is an enhanced version of 'TuxKart' featuring Tux and other Open Source Mascots. Very similar to the Super Mario Kart game for Super Nintendo but with much better graphics. Still under development!
PlanetPenguin Racer, or PPRacer as it is called for short, is a simple OpenGL racing game featuring Tux, the Linux mascot. The goal of the game is to slide down a snow and ice-covered mountain as quickly as possible, avoiding the trees and rocks that will slow you down.