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.
Typora - A minimal Markdown reading & writing app.
Cleartext - A text editor that allows only the 1,000 most common words
StackEdit - Full-featured, open-source Markdown editor based on PageDown, the Markdown library used by Stack Overflow and the other Stack Exchange sites.
Quill - Powerful, API-driven rich text editor
MarkText.app - Mark Text is a Markdown editor for Mac, Windows and Linux.