Penflip - Typewrite is a real-time collaborative writing environment with Markdown support, versioning and Dropbox syncing. Alternatives to Penflip for Web, Windows, Mac, Linux, iPhone and more.
Markdown by DaringFireball - Text-to-HTML conversion tool/syntax for web writers, by John Gruber
Pine - A lightweight, modern macOS markdown editor written in Swift
Typora - A minimal Markdown reading & writing app.
EME - Elegant Markdown editor
StackEdit - Full-featured, open-source Markdown editor based on PageDown, the Markdown library used by Stack Overflow and the other Stack Exchange sites.