"Racer" is the best free car simulator by Ruud van Gaal.
In Racer, it's really easy to make tracks and cars on your own. Lots of people are creating their own tracks and cars in Official Racer Forum. There is a huge number of files on the Internet (hundreds of cars and tracks!) and the new cars/tracks are still being produced.
Followings are not my creation. I'm mirroring these files with permission from NightEye, Prometheus and Kevin West.