BEPUphysics is a 3D physics library by BEPU. It’s fast and has a bunch of cool features like constraints, terrain, static and instanced meshes, continuous collision detection, custom collision rules, vehicles, easy multithreading, yadda yadda yadda. Full support for XBox360 and WP7 too!
The engine's primary version is for XNA, but there is a SlimDX fork, a SharpDX fork, and a dependency free fork if you don't want to use XNA. There's also some community-contributed forks!
Just starting out? Check out the Getting Started tutorial in the documentation section.
For discussions about BEPUphysics, please head to the BEPUphysics forums.
If you're looking for more BEPU-related stuff, head to the main BEPU website.
If you're feeling angelic, you can throw money at us: