OpenSteerDotNet is a port of Craig Reynolds' steering behaviour library OpenSteer.
From OpenSteer's description:
"OpenSteer provides a toolkit of steering behaviors, defined in terms of an abstract mobile agent called a "vehicle." Sample code is provided, including a simple vehicle implementation and examples of combining simple steering behaviors to produce more complex behavior. OpenSteer's classes have been designed to flexibly integrate with existing game engines by either layering or inheritance."
It is Mono compatible and has been successfully used in Mogre and Unity. It should work fine with XNA Game Studio Express, including the XBox 360 runtime.
It also contains portions of Mogre.