Markdown Here - Google Chrome, Firefox, and Thunderbird extension that lets you write email in Markdown and render...
Markdown by DaringFireball - Text-to-HTML conversion tool/syntax for web writers, by John Gruber
Dillinger - joemccann has 95 repositories available. Follow their code on GitHub.
Typora - A minimal Markdown reading & writing app.
The Most Dangerous Writing Prompts - Don't stop typing, or all progress will be lost.
StackEdit - Full-featured, open-source Markdown editor based on PageDown, the Markdown library used by Stack Overflow and the other Stack Exchange sites.