We have been shipping the eXist XML database with PresentationServer for quite some time now, but have only started using eXist on some of the "real-life" projects we are doing for our clients about 6 months ago. eXist is open source (under the LGPL license, the same license we use for PresentationServer), lightweight, supports XQuery, has been selected as one of the technology of the year 2006 by InfoWorld, and is a very popular XML database amongst people doing form applications with PresentationServer.
With all this, eXist stands as a solid XML database. Until recently, the PresentationServer processors you use to connect to eXist were not documented and figuring out how they worked was entirely left as an exercise to developer. Our documentation about eXist now lists the 4 processors you can use to query, update, delete, and insert data in eXist. Also, the new how-to section gives a few "tips and tricks" that you might find useful, as for instance how to run queries in eXist from the command line. What are your favorite "tips & tricks" for eXist? Share it with the community by posting comment here, or on the ops-users list!