Michel Fortin just announced a new object-oriented version of PHP-Markdown, making it much easier to extend the syntax for fun and no profit.
I whipped up some quick footnote support.1
It’s bundled as a WordPress plugin at the moment for extra convience, but it’s not hard to strip the plugin cruft.
Update: PHP-Markdown-Extra 1.1b1 includes footnote support out of the box. Hooray!
Because footnotes are awesome, and the only thing I want from Markdown that it doesn’t already provide. Yay! ↩