I've been fiddling with a syntax coloring hack for PHP that makes use of PHP's highlight_string function. Coupled with an upload script, it can highlight a file and stick it in a div that you can paste into a page.
The clever part was figuring out how to make the resulting code valid, as well as getting rid of non-breaking spaces (which turn into nasty invisible script-breaking beasts when you copy and paste from the browser). Just to be clear, 99% of the cleverness on my part was obtained from reading through the PHP.net user comments and public CVS.
Anyhow, here it is:
http://www.metalgeek.com/archives/2004/03/30/000018.php. And there's a link to the source as well, which shows you what it does.
I'd be interested to know if anybody finds it useful, or has any helpful comments. It would also be pretty cool to turn it into an MT plugin, but I can't quite grasp the syntax yet.