1. eXist, http://exist.sourceforge.net/
2. Extensible Markup Language (XML) 1.0, 5th Edn., http://www.w3.org/TR/REC-xml/
3. Galax, http://www.galaxquery.org
4. Leo, http://leo.saclay.inria.fr/
5. QizX Free-Engine-3, http://www.xmlmind.com/qizx/free_engine.html