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