ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.
Docsie - Docsie makes it easy to create and maintain your website documentation, without having to deploy it...
GitBook - Modern Publishing, Simply taking your books from ideas to finished, polished books.
ReadMe - A collaborative developer hub for your API or code.
Natural Docs - Natural Docs is an open-source documentation generator for multiple programming languages.
Doxygen - Generate documentation from source code