This is the source for the simplest and slickest online Markdown editor, available online here: http://markdown.pioul.fr
Just write Markdown and see what it looks like as you type. And convert it to HTML in one click.

The Minimalist Markdown Editor project is available both online as a web app, and offline as a Chrome app:

• Web app (the source code is in this repo)
• Chrome app (GitHub repository)

The two project branches share a decent amount of code. This common source code sits in /editor in each of these branches.
It's synced in and out of them using subtrees and a dedicated repository.

Unless you really want to dig in and make a lot of changes both in /editor and the rest of the app, you shouldn't need to care about this. Fork one of the two projects and send a pull request whenever you feel like it.