Fabio Subspace Policarpo released source code to a 3D collision system. This collision gives you the ability to move around your world with Quake like movement using arbitrary meshes in XNA. Subspace also released a nice Releif Mapping Shader. Releif Mapping is similar to Parallax Mapping however it uses much less GPU. With several publications and a slew of releif mapping implementations, Subspace is definately the expert on the subject.