Doxygen - Generate documentation from source code
HelpNDoc - What is HelpNDoc ? Easy to use yet powerful help authoring environment which can generate various documentation formats from a single source.
Sandcastle - Sandcastle produces accurate, MSDN style, comprehensive documentation by reflecting over the source...
HelpSmith - HelpSmith is a help authoring tool to create CHM HTML help files, manuals, and PDF documents.
NDoc - NDoc generates class library documentation from .
Manula - Manula is an online software for creating user manuals.