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.
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MarkText.app - Mark Text is a Markdown editor for Mac, Windows and Linux.
Dillinger - joemccann has 95 repositories available. Follow their code on GitHub.
Minimalist Markdown Editor - This is the simplest and slickest Markdown editor.