Erica Sadun asked, “is that a markdown?” Yes, it’s a markdown. Ba-dum ching.
In case you haven’t seen it, Marked is my app for previewing Markdown and MultiMarkdown as rendered HTML. Marked watches the file you’re editing in any text editor and updates the preview after every save. It can handle custom CSS (four built-in styles), custom processor (Textile, Wiki, GitHub flavored Markdown and more) and plenty of features especially for writers. More information on the Marked website.