I've posted the first alpha of XQuisitor, a simple open source GUI for XQuery written in Java that I demoed last week at the New York XML SIG. The hard work of implementing XQuery was done by Michael Kay. XQuisitor is just a layer of GUI icing on top of the Saxon cake.
XQuisitor is still rough around the edges, but I think it's useful enough for anyone who wants to explore XQuery. I find it makes experimenting significantly easier than using the Saxon command line interface. Bug reports, quibbles about the user interface, and so forth are appreciated. The GUI is written in Swing.
It's mostly been tested on Linux and a little on Mac OS X 10.2. It should run on any Java 1.4 platform, but I haven't verified that. Java 1.4 and Saxon 7.8 are required. XQuisitor is published under the GPL.