Base-X
BaseX "is a native, open-source XML database and efficient XPath/XQuery processor, including support for the latest Full Text and Update Recommendations. It supports very large XML instances and offers a highly interactive frontend. BaseX is written in Java and freely available for download. It is developed by the Database and Information Systems Group at the University of Konstanz." (from the web page)
The project page is located at http://basex.org/. (Using tinyURL hack to get around not being allowed restricted words ;-) )
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