PageDown
This is a modified version of PageDown, the Markdown editor used on Stack Overflow and the other Stack Exchange sites.
It includes a converter that turns Markdown into HTML, a Markdown editor with realtime preview of the generated HTML, and a few useful plugins, e.g. for sanitizing the generated HTML according to a whitelist of allowed tags.
The Markdown converter can be used both in the browser (usually in conjunction with the editor, to display a real time preview), and on the server using Node.JS.