Dr.Explain - Dr.Explain software is a help file authoring tool.
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.
Slate API Docs Generator - Create beautiful, intelligent, responsive API documentation.
HelpSmith - HelpSmith is a help authoring tool to create CHM HTML help files, manuals, and PDF documents.
FastHelp - FastHelp is a Windows Help File Generator that produces online and offline documentation.