Looks like Tumblr is on a text editor roll. After that big update with styling tools for the rich text editor, it has now revamped Markdown support for traditional PCs browsers.
From the gear menu you can pick your editor, and if you start writing in Markdown you get some helpful visual styling and a Preview option to make sure you crafted everything properly. It also supports lists and other perks.
This is a great update, a long time coming. Thanks Tumblr!