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!
Dr.Explain - Dr.Explain software is a help file authoring tool.
Natural Docs - Natural Docs is an open-source documentation generator for multiple programming languages.
CHM Editor - Best solution to edit CHM files