Typora - A minimal Markdown reading & writing app.
Grammarly - Clear, effective, mistake-free writing everywhere you type.
StackEdit - Full-featured, open-source Markdown editor based on PageDown, the Markdown library used by Stack Overflow and the other Stack Exchange sites.
LanguageTool - Free proofreading tool for OpenOffice, LibreOffice, Firefox, and Chrome.
MarkText.app - Mark Text is a Markdown editor for Mac, Windows and Linux.
Editsaurus - Highlights potential pitfalls in your writing to encourage good habits.