I built some time ago a NetBeans plugin which simplifies browsing through large xml documents a bit. Just start typing a XPath expression in the text field of the TopComponent and the result of the (last) xml editor in focus are computed and printed in the textarea below. (You can open the XPath window either by using the context menu on xml files or directly via the window menu)
It uses the JDK's javax.xml.xpath package which means XPath 1.0 support.
it also supports basic auto completion:
localized error messages (in other words: XPathExpressionException.getCause().getLocalizedMessage()):
I uploaded the module to the plugin portal. Feel free to use it ;)