Sandcastle is a documentation tool for .NET code, that takes the class structure and the inline XML comments and builds either HTML or CHM files. It saves a considerable amount of time in documenting code by hand, although the documentation is only ever as good as the inline comments!
Sandcastle is a Microsoft version of nDoc, although nDoc has not been updated for almost 2 years now. Sandcastle is used to build internal .NET Framework documentation within Microsoft.
Sandcastle is a command-line only program, so if you don’t want to run 14 commands in the correct sequence, then you need a GUI. A few are available from the community, the one I like is from Eric Woodruff.
More info at the Sandcastle Wiki.
Of course, what we really need is a tool that creates full architectural diagrams from code, but more of that another day.