Doxygen - Generate documentation from source code
Help & Manual - Help & Manual is a documentation authoring tool that allows you to create a single source text and convert it into a number of other target formats.
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