Jeni Tennison : Comparing Documents [in XSLT]
This page will hold information about how to compare several XML documents with each other using XSLT and give information about their differences.
This one is for the diff weenies. Personally I could give a shit (beyond the stylesheet's value) but I'm sure people can make good use of it by pairing it up with their RSS aggregator and the W3C's public XSLT server.