Markdown by DaringFireball - Text-to-HTML conversion tool/syntax for web writers, by John Gruber
Documize - Enterprise-grade wiki and knowledge management platform
Asciidoctor - In the spirit of free software, everyone is encouraged to help improve this project.
HackMD - Fast and flexible, real-time collaborative markdown, inspired by Hackpad.
pandoc - Pandoc is a Haskell library for converting from one markup format to another, and a command-line...
ReadTheDocs - Spend your time on writing high quality documentation, not on the tools to make your documentation work.