From LQWiki

db4o is an Open Source object database available for Java and for .NET, including Mono and CompactFramework. Features include ACID transactions, automatic class schema recognition, query-by-example, S.O.D.A. object querying API, callback methods, memory files, cascade-on-delete, multi-threaded access, and Java Webstart and servlet support.

Homepage http://www.db4o.com