Doxygen - Generate documentation from source code
Dr.Explain - Dr.Explain software is a help file authoring tool.
DocFX - A documentation generation tool for API reference and Markdown files!
WinCHM - Creating CHM file, Make CHM file, Web Help. WinCHM Pro - Help authoring software (CHM, WebHelp, DOC, PDF) . With this powerful tool you can not only create chm file from scratch, but also convert a set of existing HTML files to a chm file.
Natural Docs - Natural Docs is an open-source documentation generator for multiple programming languages.
Document360 - Self-service knowledge base software focused on SaaS Products & Enterprise Software Projects.