Oracle has released Berkeley DB XML 2.3.8, an open source "application-specific, embedded data manager for native XML data" based on Berkeley DB. It supports the recent proposed recommendations of XQuery 1.0 and XPath 2.0. It includes C++, Java, Perl, Python, TCL and PHP APIs. According to the announcement, "This release of Berkeley DB XML improves many aspects of query planning and execution. Using indexed node storage, users will generally experience significant speed increase. A new event-style layer allows for tight coupling between Berkeley DB XML and other XML processing code. This greatly enhances integration with programming languages and XML parsing libraries by eliminating the need to create and then re-parse XML content. The W3C XQuery 1.0 specification is nearing completion and this release of Berkeley DB XML is compliant with the current Proposed Recommendation."
I've never really played with this product, but I have a funny feeling I'm going to be looking at it a lot more closely in 2007. Berkeley DB XML is published under a custom, viral license that is compatible with most major open source licenses.
