Fatdog have released XML Query Engine v0.99, which implements the W3C's XML Query language.

The free evaluation version, which also implements XQL, contains limited support for the developing XQuery language. Howard Katz of Fatdog writes:

This is a first cut at a very limited implementation of the full XQuery grammar. At this point much more of the grammar remains unsupported than supported. This implementation supports FLWR expressions, element constructors, a limited range of XPath expressions on elements only, simple predicates testing element existence and text equality, and that's about it. The good news is that with the exception of expression lists, FLWRs can be explored in almost full recursive generality, and the features that are in place can be employed against actual data.

Here's a sample query that will work in XML Query Engine.

<results>
    FOR $book IN //book
        FOR $author IN $book/author
        WHERE $author/first = 'Dan' 
        RETURN
            <DanTheMan>
                $author
            </DanTheMan>
</results>

XML Query Engine is a commercial product. More information is available from Fatdog.