Doxygen - Generate documentation from source code
Documize - Enterprise-grade wiki and knowledge management platform
Sandcastle - Sandcastle produces accurate, MSDN style, comprehensive documentation by reflecting over the source...
HackMD - Fast and flexible, real-time collaborative markdown, inspired by Hackpad.
NDoc - NDoc generates class library documentation from .
ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.