1)   ObjectDB for Java/JDO - Free Edition 1.00
<a href='http://www.objectdb.com'>ObjectDB for Java/JDO</a> is a powerful yet compact Object Database for Java written entirely in Java. <a href='http://www.objectdb.com/database/jdo/free-download/'>Free Download</a> is available.
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 2)   db4o for Java 5.2
db4o (database for objects) is the open source object database. Features include native Java object persistence, object-oriented replication, automatic schema recognition, 400K footprint. Ideal for embedded use and in realtime systems.
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 3)   db4o 4.0
db4o (database for objects) is the open source object database for Java and .NET. Features include: fast native object persistence, ACID transactions, query-by-example, S.O.D.A. object query API, automatic class schema recognition, small footprint.
db4o, Database, Objects, object, OO, .NET, dotNet, C#, csharp, Mono,