I've created a new tool to assist with developing in XML. This is capable of taking an arbitrary XML document, and performing a variety of tasks on it.
At the moment, it can beautify, or pretty-print, a file, apply an arbitrary xPath and show the results, and apply an arbitrary XSL transformation.
All processing is done by the browser (there is no server-side component to this at all). Because of this, results will vary slightly by browser, since each browser flavor has idiosyncrasies involving their respective XML / XSL engines.
It works in IE6+, Firefox, Opera, Safari, and Chrome. It was quite fun to make, and I'd love to hear suggestions for future improvements!
The tool itself can be found here: http://chris.photobooks.com/xml/default.htm