How to format Visual Studio XML documentation for display on the web
Check out Sandcastle.
Sandcastle produces accurate, MSDN style, comprehensive documentation by reflecting over the source assemblies and optionally integrating XML Documentation Comments.
Wikipedia has more information and links to additional projects that integrate with sandcastle.
Another option (which appears to be much easier to use) is docu:
A documentation generator for .Net that isn't complicated, awkward, or difficult to use. Given an assembly and the XML that's generated by Visual Studio, docu can produce an entire website of documentation with a single command.
A third option is NDoc:
NDoc generates class library documentation from .NET assemblies and the XML documentation files generated by the C# compiler (or with an add-on tool for VB.NET).
NDoc uses pluggable documenters to generate documentation in several different formats, including the MSDN-style HTML Help format (.chm), the Visual Studio .NET Help format (HTML Help 2), and MSDN-online style web pages.
Check out Sandcastle Help File Builder. This is a GUI tool that makes Sandcastle pretty easy to work with. It will install to a folder in Start Menu->Program Files->Sandcastle Help File Builder.; It's pretty easy to use, but it's not incredibly fast.
Hope that helps!