Typora - A minimal Markdown reading & writing app.
MacDown - MacDown is an open source Markdown editor for OS X, released under the MIT License.
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.
Pine - A lightweight, modern macOS markdown editor written in Swift
Markdown by DaringFireball - Text-to-HTML conversion tool/syntax for web writers, by John Gruber