Typora - A minimal Markdown reading & writing app.
StackEdit - Full-featured, open-source Markdown editor based on PageDown, the Markdown library used by Stack Overflow and the other Stack Exchange sites.
Markdown by DaringFireball - Text-to-HTML conversion tool/syntax for web writers, by John Gruber
MarkText.app - Mark Text is a Markdown editor for Mac, Windows and Linux.
Gingko - Write and organize your ideas in Gingko tree documents.
iA Writer - Minimal Design, Maximum Focus