Dr.Explain - Dr.Explain software is a help file authoring tool.
Documize - Enterprise-grade wiki and knowledge management platform
MadCap Flare - Documentation for Any Audience, Language or Format
ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.
HelpNDoc - What is HelpNDoc ? Easy to use yet powerful help authoring environment which can generate various documentation formats from a single source.
HackMD - Fast and flexible, real-time collaborative markdown, inspired by Hackpad.