There are a few tools for building javadoc-like documentation for .NET code available out there on the ‘net. Unfortunately, the major contenders (e.g. NDoc, Sandcastle) suffer from a few flaws: they are variously not free (gratis), not free (libre), not cross-platform, not maintained, and/or not easy-to-use. Therefore:

Presenting: NDocProc.

Download a zip snapshot from here, or use mercurial to check out the repository:

hg clone http://hg.opensource.lshift.net/ndocproc

Update: New release available.