I've just put an XML DDR Query tool online. Its built on the SimpleXML parser using PHP. I haven't set it up as a web service, it's bundled for you to download and install (presumably) onto your own machine and run on localhost.
The XML DDR Query tool allows you to delve into your FileMaker databases in a way that isn't possible using FileMaker's built in functions. It's primary distinguishing factor is that it is fast. Other excellent tools already exist. They are great tools and they aim to be complete but they take some time to load a DDR. I found that the initial load time prevented me from doing ad-hoc work which would really benefit from DDR analysis. My XML DDR Query tool will answer your questions in less than the time it takes to load a DDR into other tools. I wanted to be able to dump the DDR to ask a single question without waiting an hour for it to be processed. I wanted quick answers. The XML DDR Query tool is my response to that need. Its not a 100% solution but it answers most of my questions, most of the time, in no time at all. It could help you too.