LightPaper - Admired by thousands, LightPaper is a powerful markdown editor with features such as Multi-Tabs, Folder Navigator, Distraction Free Mode, Quick Open, Live Preview, Full GFM Support, Math rendering etc.
Markdown by DaringFireball - Text-to-HTML conversion tool/syntax for web writers, by John Gruber
Markdown Live Preview - Stop guessing about your Markdown formatting 📝
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.
Joplin - Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, tagged and modified either from the applications directly or from your own text editor.