XQuery engine IPSI-XQ
If you're itching to get going with XQuery and don't have the Yukon bits, here's an interesting stack to try. This is also one of the first public releases I've seen of such a stack that has last November's working draft explicitly integrated. Note that its Java based thing, so you might need to install Sun's VM.
http://www.ipsi.fraunhofer.de/oasys/projects/ipsi-xq/index_e.html