SuperTuxKart is a fun & casual racing game in the spirit of Mario Kart. It has a long history after being originally a fork of a game called TuxKart, but has ultimatly emerged as a Free Software reference title. This year it also hosted serveral students under the Google Summer of Code event, which greatly enhanced a few previously lacking areas (shaders, multiplayer etc.).
SuperTuxKart is based on the Irrlicht engine, but its source-code is availble onder the GPLv2 or later. All art assets are released under licenses compatible to the CC-by-SA.