I downloaded the db4o open source object database and started experimenting with it on a recent Struts 2 web application. After writing JDBC and Hibernate for many years, I was in disbelief when I discovered my object graphs were being saved without any XML files or configuration! I had my objects persisted in less code than it would take to initialize an ArrayList!
Let me know if you have experience with this database. I think it was created for mobile computing, but it is serving my EE application very nicely. It has been a real breath of fresh air.
P.S. I added a couple properties to an object this morning and figured this would break everything. When I moved the new object into production, I figured I would be repairing collateral damage the rest of the day. In fact, there were no problems and I now have the time to blog about it.
http://strutsschool.blogspot.com/2007/11/saving-objects-with-db4o.html
Scott Stanlick (coauthor of Struts 2 in Action)