I notice that the only “online” checker in the answers seems a bit clunky. It requires submission to a server, and it doesn’t handle namespaces properly.
I thought I could do better, and that such a tool would be quite useful, so I made one. I realize it is slightly self-promoting to mention it here, but it does specifically answer the question, I think!
http://chris.photobooks.com/xml/default.htm
Mine can apply an xpath expression to an arbitrary XML document and display the results. It can also beautify, or pretty-print an XML document, and apply an XSLT transformation to it. All processing is done by the browser, and it should work in IE6+, Firefox, Opera, Chrome, and Safari.
Please let me know if you encounter any issues with it!