Typora - A minimal Markdown reading & writing app.
Markdown by DaringFireball - Text-to-HTML conversion tool/syntax for web writers, by John Gruber
Haroopad - Haroopad is a markdown enabled document processor for creating web-friendly documents.
Visual Studio Code - Build and debug modern web and cloud applications, by Microsoft
StackEdit - Full-featured, open-source Markdown editor based on PageDown, the Markdown library used by Stack Overflow and the other Stack Exchange sites.
MarkText.app - Mark Text is a Markdown editor for Mac, Windows and Linux.