Today I deployed a major update to my Jazz object model which implements a full REST web interface that is currently accessible through a web browser. Visit http://www.jazztoolbox.com/ for the RDocs - there are plenty of examples at the bottom of the index file, or you can try this immediately:
Some fairly complex relationships can be explored in this manner just using your web browser (displaying XML). Note that I still do not have a whole lot of data in here; all the data is just my personal knowledge of Jazz theory. Eventually I will consult a theory reference and develop the database of jazz more thoroughly.