Greenboard - Generate beautiful static api documentation
Dr.Explain - Dr.Explain software is a help file authoring tool.
You Need A Wiki - Create a wiki with Google Docs
HelpSmith - HelpSmith is a help authoring tool to create CHM HTML help files, manuals, and PDF documents.
Docusaurus - Easy to maintain open source documentation websites
MadCap Flare - Documentation for Any Audience, Language or Format