I haven't actually used XPath in a project yet, so I was only somewhat familiar with it. I just saw this great XPath tutorial while checking out the JXPath project on Jakarta. It's very nice - quick and simple. No fluff, just the facts.
The site, W3Schools.com, also has a bunch of other tutorials available for all types of XML and Web technologies.
Good resource. Very nice for a busy developer.
P.S. The site even has a good sense of humor.