MacDown - MacDown is an open source Markdown editor for OS X, released under the MIT License.
Typora - A minimal Markdown reading & writing app.
MarkNote - A simple markdown note management app.
Markdown by DaringFireball - Text-to-HTML conversion tool/syntax for web writers, by John Gruber
Zettlr - Write Markdown documents with a comprehensive GUI and many workflow/time management tools.
Minimalist Markdown Editor - This is the simplest and slickest Markdown editor.