Doxygen - Generate documentation from source code
HelpNDoc - What is HelpNDoc ? Easy to use yet powerful help authoring environment which can generate various documentation formats from a single source.
DocFX - A documentation generation tool for API reference and Markdown files!
Adobe RoboHelp - Adobe RoboHelp empowers organizations to deliver engaging help and policy content across mobile devices, formats, and platforms.
Natural Docs - Natural Docs is an open-source documentation generator for multiple programming languages.
HelpSmith - HelpSmith is a help authoring tool to create CHM HTML help files, manuals, and PDF documents.