BEPUphysics is a 3D physics library by BEPU. It’s fast and has a bunch of features. Windows (XNA, SlimDX, SharpDX), XBox360, Windows Phone, and cross-platform dependency free versions are available.

Box2D.XNA is a C#/XNA port of Box2D (http://code.google.com/p/box2d). It is optimized for the Xbox 360, minimizing allocations per frame and CPU time. Both the core engine and all test cases are ported including an XNA renderer.

The Farseer Physics Engine is an easy to use 2D physics engine that focuses on simplicity, useful features and enabling the creation of fun and dynamic games.

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.