EpicEditor is an embeddable JavaScript Markdown editor with split fullscreen editing, live previewing, automatic draft saving, offline support, and more. For developers, it offers a robust API, can be easily themed, and allows you to swap out the bundled Markdown parser with anything you throw at it.
the old public-api doesn't exist anymore, new api needs auth. Links in the form of twitter.com/[username]/profile_picture do work.