The dbXML project has an interesting history. Some have compared it to a soap opera. Though there has been quite a bit of flux in the project, its goal has been to produce a high quality, small footprint XML database that just works.
dbXML is a native XML database written in Java. Native XML databases (NXDs) are databases that store XML using an internalized format for faster overall processing and representational flexibility. NXDs also provide support for indexing XML for improved query performance.
The article continues at http://www.xml.com/pub/a/2004/02/25/dbxml.html