I used that to make a demo for my realtime collaborative editing library:

http://sharejs.org/wiki/HackerNews

The rendered markdown updates live for you and everyone else viewing the page!

Showdown's a great library, I use it with some tweaks on http://notepag.es/ for doing markdown/latex math editing. Given it is just a chain of regexes though, modifying it to add new functionality can be a little hairy.

Reminds me of http://showdown.im , which I've been using up until recently. I know they both use the same library, but this one looks like it has syntax highlighting.

Based on the underlying library, I created this wysiwyg-ish editor a while back: http://troelskn.github.com/upflow/

Nice... unlike the Mac OS X app on HN earlier, this doesn't scroll the preview to the top when the markdown is changed.

That's awesome! I want to work on something that will convert notes in Catch.com with #markdown using that.