The Modis Team has released Sedna 2.2, an open source native XML database for Windows and Linux written in C++ and Scheme and published under the Apache License 2.0. Sedna supports XQuery and its own declarative update language. This release fixes bugs, adds XQuery triggers, and no longer requires root privileges to run on Linux.
Of the open source XML databases, this is the one I know the least about. Anyone want to comment on this one?