AquaQuery is based on Apple's excellent NSXML and WebKit toolkits. XQuery support is provided, of course, by NSXML.
Usage is simple and straightforward. Compose or paste your XML source document in the "Source" tab. Compose your XQuery in the "Query" tab. Click the "Query" button in the toolbar and view the results in the "Result" tab. The result tab includes both the raw serialization of the result as well as a WebKit-rendered view that may sometimes be useful when using XQuery to produce and XML, XHTML or HTML result.