ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.
Doxygen - Generate documentation from source code
Docsie - Docsie makes it easy to create and maintain your website documentation, without having to deploy it...
Sandcastle - Sandcastle produces accurate, MSDN style, comprehensive documentation by reflecting over the source...
ReadMe - A collaborative developer hub for your API or code.
DoxyS - DoxyS is a code documentation tool for C++/C.