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From freshmeat.net]
db4o (database for objects) is a client/server object database, available in Java and in C#. It features transactions, automatic class recognition, query-by-example, callback methods, memory files, cascade-on-delete, multi-threaded access, and Java Webstart and servlet support.
The article continues at
http://freshmeat.net/projects/db4o/?branch_id=1733&release_id=154567