Markdown by DaringFireball - Text-to-HTML conversion tool/syntax for web writers, by John Gruber
Typora - A minimal Markdown reading & writing app.
GitHub Flavored Markdown - GitHub uses "GitHub Flavored Markdown," or GFM, across the site--in issues, comments, and...
StackEdit - Full-featured, open-source Markdown editor based on PageDown, the Markdown library used by Stack Overflow and the other Stack Exchange sites.
Texy! - Texy! is a lightweight markup language as well as converter of this format to XHTML, in a form of a...
MarkText.app - Mark Text is a Markdown editor for Mac, Windows and Linux.