ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.
Documize - Enterprise-grade wiki and knowledge management platform
Docsie - Docsie makes it easy to create and maintain your website documentation, without having to deploy it...
HackMD - Fast and flexible, real-time collaborative markdown, inspired by Hackpad.
ReadMe - A collaborative developer hub for your API or code.
Doxygen - Generate documentation from source code