XOM 1.1 bundles Jaxen 1.1.
Bob McWhirter talked about Jaxen at Software Development 2003 West.
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.