I don't know if anyone else has heard of this "game," but it looks very fun, especially if multiple people can interact. There is no plot, and no end game goal, but the physics involved representing vehicular interaction, damage representation and simulation, and terrain interaction are astounding. The game itself is free and open-source, and can be downloaded from here.

To see what it is about, here is a short intro clip:

[youtube]http://www.youtube.com/watch?v=3A6OHnAD_Pc[/youtube]

Looks pretty cool.

its like the minecraft of vehicular physics

edit: lolwow! all that in a 41mb download + content packs if you want em