I've been moving more and more resources over to xml and I'm still being impressed with the results. Previously I've used XML-like files (with a similar tag structure, but much simpler) which was good but I wrote my own little parsers which didn't help too much when new data needed adding. Now I'm using a proper xml parser (Java's built-in DOM parser) and its much easier to work with and extend.
Somewhat unrelated, I've also been looking at XSL. XSL is basically an XML file which accepts an XML file and transforms it into another XML file. Rather cool idea, as you can convert a generic xml data file into XHTML with proper formatting and everything. However XSL has to be well formed XML itself, leading to one of the most ugly and special-case ridden half-programming languages I've ever seen. Ugh. The phrase "write only language" wouldn't be too far wrong I feel.