Typora - A minimal Markdown reading & writing app.
Lenote - Lenote is a simple, distraction-free notetaking app that lets you capture and organize thoughts more productively.
iA Writer - Minimal Design, Maximum Focus
Letterspace - A simple note-taking app on iOS & Mac
Haroopad - Haroopad is a markdown enabled document processor for creating web-friendly documents.
StackEdit - Full-featured, open-source Markdown editor based on PageDown, the Markdown library used by Stack Overflow and the other Stack Exchange sites.