Wolfgang Meier et al have released eXist DB 1.2.6, an open source XQuery database:
It stores XML data according to the XML data model and features efficient, index-based XQuery processing.
eXist-db supports many (web) technology standards making it an excellent application platform:
- XQuery 1.0 / XPath 2.0
- XSLT 1.0 (using Apache Xalan) or XSLT 2.0 (optional using Saxon)
- HTTP interfaces: REST, WebDAV, SOAP, XMLRPC, Atom Publishing Protocol
- XML database specific: XMLDB, XQJ/JSR-225 (under development), XUpdate, XQuery update extensions (to be aligned with the new XQuery Update Facility 1.0
eXist is probably the major pure XML open source (LGPL) native XML database. I've been wanting to spend some serious time with this product for a while now, but haven't had the opportunity. From what I hear, these days it's adequate for single server systems. I'd be interested to hear what folks have built on top of it.