ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.
Doxygen - Generate documentation from source code
GitBook - Modern Publishing, Simply taking your books from ideas to finished, polished books.
DocFX - A documentation generation tool for API reference and Markdown files!
NDoc - NDoc generates class library documentation from .
Wikidot - Online collaborative wiki hosting for organizations,etc.