Open Source

eXist
eXist is an Open Source repository and retrieval engine for XML documents.
http://exist.sourceforge.net/

interBasis (IB 2.x)
SGML/XML-savy structured fulltext engine and multi-protocol information management system. (BSn)
http://www2.bsn.de/Z39.50/

Lore
Lore is a database management system (DBMS) for XML. The Lore executables are available for public use.
http://infolab.stanford.edu/lore/

Xindice
DBMS written in Java to optimize the storage, retrieval, querying, and management of XML-based documents. Xindice was known as "dbXML Core" prior to joining the Apache XML Project.
http://xml.apache.org/xindice/