HelpNDoc - What is HelpNDoc ? Easy to use yet powerful help authoring environment which can generate various documentation formats from a single source.

Doxygen - Generate documentation from source code

ManualsLib - Just enter the keywords in the search field and find what you are looking for!

Sandcastle - Sandcastle produces accurate, MSDN style, comprehensive documentation by reflecting over the source...

Manuzoid - Repository that includes user manuals for various products

Javadoc - Oracle Technology Network is the ultimate, complete, and authoritative source of technical information and learning about Java.