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.

 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.

 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.