GitBook - Modern Publishing, Simply taking your books from ideas to finished, polished books.
MkDocs - Project documentation with Markdown.
Docusaurus - Easy to maintain open source documentation websites
Doxygen - Generate documentation from source code
Hyperbook - Hyperbook is a quick and easy way to build interactive workbooks, that support modern standards and runs superfast.
ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.