JPOX is a free and fully compliant implementation of the JDO1, JDO2, JDO2.1 and JPA1 specifications, providing transparent persistence of Java objects. It supports persistence to all of the major RDBMS on the market today, persistence to the DB4O object datastore, supporting all of the main Object-Relational Mapping (ORM) patterns demanded by today's applications, allows querying using either JDOQL, SQL, or JPQL, and comes with its own byte-code enhancer. JPOX is available under the Open Source Apache 2 license, allowing access to not just a top quality Java persistence implementation but also to the source code, allowing you to contribute to the success story of the principal standards-compliant Open Source persistence implementation in the world today.

JPOX 1.0 is JDO1 compliant.
JPOX 1.1 is JDO1/JDO2 compliant, and is the JDO2 Reference Implementation.
JPOX 1.2 is JDO1/JDO2/JDO2.1/JPA1 compliant, and allows persistence to DB4O datastores.

JPOX provides a high performance persistence mechanism, outperforming some other O/R mappers using independent publically available benchmarks. What more reason do you need to download it today and see the benefit? Download JPOX Data Sheet.

Please note that development of JPOX is finished, and future direction is taking place under the name of DataNucleus and in particular DataNucleus Access Platform