Doxygen - Generate documentation from source code
ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.
Sandcastle - Sandcastle produces accurate, MSDN style, comprehensive documentation by reflecting over the source...
Docsie - Docsie makes it easy to create and maintain your website documentation, without having to deploy it...
NDoc - NDoc generates class library documentation from .
ReadMe - A collaborative developer hub for your API or code.