This is the simplest and slickest Markdown editor. Just write Markdown and see what it looks like as you type.
The Minimalist Markdown Editor also comes with one-click HTML conversion, file support, persistent tabs, and other discreet gems.
MME is privacy-friendly, open-source, and works totally offline.
The editor is also available online as a web app, although with no file/tab support: http://markdown.pioul.fr