Check out this screen shot of an XPath Tool Window running inside Visual Studio 2005 that executes XPath queries against your current XML document window. DonXML as written a nice intro to how it works on his website and has posted the installer and source code on the new Mvp.Xml Codeplex Project. The code shows how you can integrate with the XML editor in Visual Studio and I'm sure this nifty tool will help a lot of users who are struggling with getting their XPath expressions working. Nice work Don!
Daniel Cazzulino wrote a nice piece on Goto Definition in the XML Editor!