Dr.Explain - Dr.Explain software is a help file authoring tool.
Doxygen - Generate documentation from source code
HelpSmith - HelpSmith is a help authoring tool to create CHM HTML help files, manuals, and PDF documents.
Slate API Docs Generator - Create beautiful, intelligent, responsive API documentation.
MadCap Flare - Documentation for Any Audience, Language or Format
FastHelp - FastHelp is a Windows Help File Generator that produces online and offline documentation.