Henge3D is a 3D physics library written in C# for XNA. It is implemented entirely in managed code and is compatible with the XBOX 360. It has many features and its also multi-threaded. It runs okay on PC, however its slow on Xbox 360. Hengd3D is FREE!
-Rigid body simulation with collision detection, response, and approximated friction.
-Supports collision skins consisting of arbitrary convex polyhedra, spheres, capsules and planes. Collision detection against triangle meshes is also supported.
-Supports a number of constraints, including body point constraints, revolute joints, universal joints, prismatic joints, etc.
-Multi-threaded collision detection and collision response.
-Ability to import collision skins from Blender and potentially other modeling programs.
-Supports ragdolls using collections of rigid bodies with constraints (see the Holodeck program for examples).