We write every post for The Changelog in Markdown. Needless to say, we love markdown, and loath the moment we ever have to write a post or author online content without it, say for example a WYSIWYG editor. Bah, lame. Forget it.
EpicEditor is a very nice embeddable JavaScript Markdown editor by Oscar Godson. Check out the homepage for some detailed API and theming docs as well as a live example to play with.