Not sure when they added it (seems like just today), but [Tumblr](http://www.tumblr.com/) has just added Markdown support for posts. Huzzah! My [Rathole](http://rathole.tumblr.com/) tumblog is hosted by Tumbler, and this is just one more reason to love ’em.
What’s a “tumblog,” you say? Check out the grandaddy of them all, [project.ioni.st](http://project.ioni.st).