Rig of Rods is an interesting piece of software. It’s an open source car/plane/boat simulator with a number of user-made vehicles and areas you can explore. The vehicles themselves, though, use “soft-body physics” – the wheels and chassis are flexible real-time objects that respond to the environment. A collision can deform them. The flight simulator uses advanced blade physics and the boating uses bouyancy calculations for realistic hull simulation.
Beyond that, there are currently over 2000 user mods (it’s been around a couple years so the library is built up). Being open source, you can go crazy building your dream vehicle and testing it out. I like that a lot.
I also like that it’s free. And it works on Mac.
And I LOVE that you can rock crawl.
Download it here: Rig of Rods.