XMLGet is an efficient, open source query tool for XML documents based on the XQL query language
You can download the Java source and binaries for the latest version (1.0) of XMLGet here
You can also download the user documentation here
A document entitled "XMLGet -- Testing, Profiling, Benchmarking, and Proposed Future Work" can be downloaded here. This describes some of the caveats found in XMLGet during testing, profiles various sections of the XMLGet parser, compares performance against other XQL engines, and makes suggestions for future enhancements to XMLGet.