XOM 1.1 bundles Jaxen 1.1.
Added the slidedeck from Bob McWhirter's SD-West presentation.
Check out these Performance Benchmarks comparing dom4j and Jaxen against Xerces and Xalan.
Sun chooses Jaxen for the XPath engine JSTL and Web Services pack
Since the reference implementation of Java API for XML Messaging is based on dom4j and Jaxen, that means you can use Jaxen to query SOAP messages on the Java platform too!
Check out Elliotte Rusty Harold's book chapter on XPath and Jaxen
Alex Chaffee wrote XPath Explorer to help visualize results of XPath expressions.
You can download the current 1.0-FCS release or the latest beta of 1.1 via the releases page.