SuperTuxKart is a completely free kart racing game featuring the mascots of many open source projects. STK itself is open source and students and hobbyists are encouraged to experiment with it. You can make your own karts and tracks in Blender and use them in game. There is an active community that is constantly making new additions and refinements to the game. STK has been actively developed for over 8 years by our team. If you are a C++ programmer, an artist, translator, musician, or sfx designer you can get involved in the project.
Neon_Knight says
I've been playing since 0.6.2. It's a very fun game which got better and better after every release. Highly recommended.