BEPUphysics (for Windows, XBox and WP7)

http://channel9.msdn.com/coding4fun/blog/Get-a-free-and-open-source-physics-library-in-BEPUphysics-for-Windows-XBox-and-WP7

“Today’s project is a physics engine, that’s free, open source and cross Microsoft platform. And as you’d expect, not only do you get this cool engine, but a wide sample of demo’s too. The scope of this engine is daunting, yet in looking at the code in the samples and the documentation, it looks very approachable too.