The car tutorial
Outdated by today's standards, but the track itself is fun to play around with and can be used to prototype a racing game. The car model is also pretty neat and there are some cool scripts.
The tutorial itself is probably a bit hard to follow now seeing as it was made for Unity 3, but I've verified that the project works in Unity 3.5.7, which is the latest version of Unity 3. You can can get this version of Unity and the project from the links on this page.
Disclaimer: This project is no longer officially supported. Most likely you'll get no support from me either, just use take it for what it is.