Jaxen: Java XPath Engine

Jaxen is a fast Java engine for XPath 1.0 that works on all XML object models. The following models are currently supported out of the box

Check out these new 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 JAXM 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 Harolds book chapter on XPath and Jaxen
Alex Chaffee wrote XPath Explorer to help visualize results of XPath expressions.

You can download the current $VERSION$ release or a nightly build via the download page.

You could try browsing the FAQ or the online JavaDoc.

Contributors are welcome to join this project. Once you've browsed the FAQ you could try sending an email to one of the mailing lists below or check out the project page.

There is a jaxen-interest email list where you can share ideas and experiences, ask for help, give us feedback or discuss your requirements.

You can also browse the archives.