Doxygen - Generate documentation from source code
Slate API Docs Generator - Create beautiful, intelligent, responsive API documentation.
Sandcastle - Sandcastle produces accurate, MSDN style, comprehensive documentation by reflecting over the source...
FastHelp - FastHelp is a Windows Help File Generator that produces online and offline documentation.
Javadoc - Oracle Technology Network is the ultimate, complete, and authoritative source of technical information and learning about Java.
Manula - Manula is an online software for creating user manuals.