Doxygen - Generate documentation from source code
Dr.Explain - Dr.Explain software is a help file authoring tool.
Sandcastle - Sandcastle produces accurate, MSDN style, comprehensive documentation by reflecting over the source...
MadCap Flare - Documentation for Any Audience, Language or Format
Javadoc - Oracle Technology Network is the ultimate, complete, and authoritative source of technical information and learning about Java.
HelpNDoc - What is HelpNDoc ? Easy to use yet powerful help authoring environment which can generate various documentation formats from a single source.