MediaWiki - MediaWiki is a free software wiki package written in PHP, originally for use on Wikipedia.
HelpNDoc - What is HelpNDoc ? Easy to use yet powerful help authoring environment which can generate various documentation formats from a single source.
TiddlyWiki - a non-linear personal web notebook
Dr.Explain - Dr.Explain software is a help file authoring tool.
Confluence - Confluence is content collaboration software that changes how modern teams work
MadCap Flare - Documentation for Any Audience, Language or Format