MkDocs - Project documentation with Markdown.
HelpNDoc - What is HelpNDoc ? Easy to use yet powerful help authoring environment which can generate various documentation formats from a single source.
GitBook - Modern Publishing, Simply taking your books from ideas to finished, polished books.
Adobe RoboHelp - Adobe RoboHelp empowers organizations to deliver engaging help and policy content across mobile devices, formats, and platforms.
Docusaurus - Easy to maintain open source documentation websites
HelpSmith - HelpSmith is a help authoring tool to create CHM HTML help files, manuals, and PDF documents.