ObjectRelationalBridge (OJB) is an OpenSource project providing an ODMG compliant ObjectRelationalMapping for Java. A JDO compliant Implementation is in the works.
The design is "pattern driven". It applies many ideas from CrossingChasms, MetaLevel architectures, MetaObjectProtocol, OnReflection, PoSa, "Mapping Objects To Relational Databases" (http://www.ambysoft.com/mappingObjects.pdf), "Applying UML and Patterns" by CraigLarman and others.
The project's homepage: http://db.apache.org/ojb/