Sleepycat Software has released Version 2.0 of their XML database. Here's what the announcement said:

Berkeley DB XML is a native XML database engine for use within your product. Made available as a C++ library with language bindings for Java, Perl, Python, PHP and Tcl Berkeley DB XML integrates directly into your application, it is not a standalone database server. Berkeley DB XML provides XQuery access into a database of document containers. XML documents are stored and indexed in their native format using Berkeley DB as the transactional database engine. Berkeley DB XML is one of the most sophisticated and reliable native XML database engine available today. The important new features and benefits of Berkeley DB XML 2.0.7 include:

• XQuery 1.0 and XPath 2.0 (July 2004 draft)
• Node or whole document based storage
• Optional document validation
• Sophisticated document indexing to speed query access
• C++, Java, Perl, Python, TCL and now PHP APIs
• Command line access to document repositories

I've used an earlier version this database in several projects and have really liked it. Its not a standalone server, so its really a great way to add lightweight persistence to an application without the set-up hassles of MySQL or some other DB server.