Dr.Explain - Dr.Explain software is a help file authoring tool.
Documize - Enterprise-grade wiki and knowledge management platform
Adobe RoboHelp - Adobe RoboHelp empowers organizations to deliver engaging help and policy content across mobile devices, formats, and platforms.
ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.
MadCap Flare - Documentation for Any Audience, Language or Format
HackMD - Fast and flexible, real-time collaborative markdown, inspired by Hackpad.