Tux Racer, a 3D computer game starring the Linux mascot Tux the penguin, was formerly free software released under the GNU General Public License. It is currently developed by Sunspire Studios at http://www.tuxracer.com and retails for $14.99. Tux racer runs on GNU/Linux, Mac OS X and Windows.
In the game, the player controls Tux or one of three other characters as he slides down a course of snow and ice collecting herring. Ice is best for gaining speed, snow helps manœuvrability and the occasional rocky patch will slow Tux down. There are also trees in the way and flags to mark out the course.
The controls are typical of a racing sim, except that the up arrow, "paddle" will slow Tux down when at a high speed and can also be used to increase the length of a jump. Jumps can be caused by the shape of the landscape or by holding down the "energy" key and releasing it. Releasing the key when a jump was imminent anyhow will naturally make a larger jump.
Players may design new courses by creating (or, more easily, modifying) three bitmap images, to indicate height, surface and objects (trees, flags, herring etc.).
Like most, if not all, formerly Open-Source games, its replay value is improved by being easily modified. The game can be quite amusing, although unrealistic, if it is altered so that not only can penguins fly, they can exceed mach one.
The old open-source versions of Tux Racer can be found at http://tuxracer.sourceforge.net.