Markable.in: Online Markdown Editor. Dropbox and Tumblr supported.
This is a neat online Markdown editor. It doesn’t sync with Dropbox though, it only imports/exports from its own folder. I hope it will be able to import from any folder in the future. The other neat feature that I, as a Tumblr user, really like is its ability to export to Tumblr. Again a drawback here, it doesn’t allow posting to sub-blogs (like MOSX). I assume this feature is coming in the future. Markable is beta!