HelpNDoc - What is HelpNDoc ? Easy to use yet powerful help authoring environment which can generate various documentation formats from a single source.
Documize - Enterprise-grade wiki and knowledge management platform
HelpSmith - HelpSmith is a help authoring tool to create CHM HTML help files, manuals, and PDF documents.
ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.
Manula - Manula is an online software for creating user manuals.
HackMD - Fast and flexible, real-time collaborative markdown, inspired by Hackpad.