BEPUphysics : 3D Physics Library
BEPUphysics is a free and open source 3D physics library. It is fast and sports features such as constraints, terrain, static and instanced meshes, continuous collision detection, custom collision rules, vehicles, easy multithreading etc. Documentation and demo projects are available that can help you get started with the library.
The engine’s primary version uses its own math libraries for cross platform development. The XNA fork makes it easier to compile for XNA-specific platforms (though it still uses the BEPUutilities math).