I was recently given the task to generate some API documentation for all of the code on a project I was working on (11 different projects, 3 different solutions) so I began looking into what was available. I remember using NDoc with old 1.1 projects, but that project was apparently abandanded. Microsoft has an October 2007 CTP release for a replacement called Sandcastle. It’s a suite of files that provide the core functionality of generating HTML and CHM files for documentation in an MSDN-like style. Apparently, this is what Microsoft uses internally to generate their MSDN documentation.
Unfortunately, Sandcastle comes with no GUI to use it but fortunately, there are several free GUIs written out there to do it for you. To find out more about it, I thought this site was most helpful.