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.
Slate API Docs Generator - Create beautiful, intelligent, responsive API documentation.
Dokit - The solution for disseminating know-how: work instructions, procedures, knowledge base, documentation techniques, product support, wiki enterprise.