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.
GitBook - Modern Publishing, Simply taking your books from ideas to finished, polished books.
Dr.Explain - Dr.Explain software is a help file authoring tool.
Docsify.js - A magical documentation site generator.
MadCap Flare - Documentation for Any Audience, Language or Format