HelpNDoc - What is HelpNDoc ? Easy to use yet powerful help authoring environment which can generate various documentation formats from a single source.
MkDocs - Project documentation with Markdown.
Dr.Explain - Dr.Explain software is a help file authoring tool.
GitBook - Modern Publishing, Simply taking your books from ideas to finished, polished books.
CHM Editor - Best solution to edit CHM files
Docusaurus - Easy to maintain open source documentation websites