I'm pretty surprised that I haven't seen more fuss about XPathMania, which is a fantastic tool put together by DonXml Demsak.
This little Visual Studio addin creates a VS tool window where a developer can write an XPath statement and test it against the current document. After running the XPath query, it will list and highlight the nodes returned from the query.
This is bliss... I've been looking for something like this for a long, long time. Great work, Don!