HelpNDoc - What is HelpNDoc ? Easy to use yet powerful help authoring environment which can generate various documentation formats from a single source.
GitBook - Modern Publishing, Simply taking your books from ideas to finished, polished books.
HelpSmith - HelpSmith is a help authoring tool to create CHM HTML help files, manuals, and PDF documents.
DocFX - A documentation generation tool for API reference and Markdown files!
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.
MkDocs - Project documentation with Markdown.