PageDown is the JavaScript Markdown previewer used on Stack Overflow and the rest of the Stack Exchange network. It includes a Markdown-to-HTML converter and an in-page Markdown editor with live preview. The largest part is based on work by John Fraser, a.k.a. Attacklab. He created the converter under the name Showdown and the editor under the name WMD. See this post on the Stack Exchange blog (http://blog.stackoverflow.com/2008/12/reverse-engineering-the-wmd-editor/) for some historical information.