Mario Kart: Source

Ambitious modders brought the original Half-Life conversion Counter-Strike to life, and it became the most played online multiplayer game of all time.
Now with tools for Half-Life 2’s Source Engine in hand, a new generation of modders is trying to recreate Mario Kart, Nintendo’s highly successful racer.

The Media Gallery has some highly polished models. And the Videos show off a little of Source’s physics engine applied to a kart.
Its a long way to go, and will hopefully end up better than GoldenEye: Source.