WriteNext - The writing application that boosts your writing. Increase focus and writing performance and reduce distractions by separating your creative writing from other activities. Let writing be the only activity you perform in the writing application.
Typora - A minimal Markdown reading & writing app.
Calmly Writer - Distraction-free text editor for Mac, Windows, Linux and ChromeOS.
StackEdit - Full-featured, open-source Markdown editor based on PageDown, the Markdown library used by Stack Overflow and the other Stack Exchange sites.
Stashany - Online notepad for developers
MarkText.app - Mark Text is a Markdown editor for Mac, Windows and Linux.