These are the libraries that I implemented into my own framework and use in every recent project.
OpenTK is an OpenGL wrapper for C#. It does a decent job implementing all of OpenGL's features.
Jitter is a 3D physics library that is not tied with a graphics library, so it is ready to use in any .NET environment.
Box2D.XNA is the most able port of Box2D i've found. Although i had to strip it off the XNA bindings to use it.
AC.AL is a not yet complete but working audio library by AstrorEnales of ac-games. It currently supports .wav and .ogg playback.