Markdown by DaringFireball - Text-to-HTML conversion tool/syntax for web writers, by John Gruber
Rapidweaver - A WYSIWYG web editor for mac.
Typora - A minimal Markdown reading & writing app.
Markdown Here - Google Chrome, Firefox, and Thunderbird extension that lets you write email in Markdown and render...
StackEdit - Full-featured, open-source Markdown editor based on PageDown, the Markdown library used by Stack Overflow and the other Stack Exchange sites.
WriteWell - Jumpstart your writing with the world's best templates.