Release builds can be found here. The current release version is 1.0.
Beta builds can be found here. The current version is 1.1 beta 5. 1.1 is a major upgrade that significantly improves jaxen's conformance to the underlying XPath specs. Even though it's still officially a beta, this release is a vast improvement over 1.0, and all users are strongly encouraged to upgrade. With a few small exceptions (e.g. the the document() function has moved to the org.jaxen.function.xslt package and the ElectricXML navigator has been deleted) 1.1 is backwards compatible with code written to the 1.0 APIs.
The latest official build is 1.0-FCS:
The recommended build is 1.1-beta-5:
Nightly builds have been temporarily suspended as we ramp up with using Maven. However, you can grab a tarball of the current CVS head from FishEye if you like. To build Jaxen, you'll need to have maven installed. Assuming you do you should be able to just type "maven jar" at the command line in the jaxen directory to create a JAR archive.