It's not common knowledge at this point but Oracle recently open-sourced a wide swath of Oracle TopLink, its object/relational persistence architecture, in the form of TopLink Essentials -- which in fact is rolled into Sun's GlassFish project. (Source is available here.)
The news here is that TopLink Essentials has been named the official Reference Implementation for the EJB3 Java Persistence API (JPA), which provides the persistence architecture within the EJB3 spec. As of yesterday you can download TopLink Essentials directly from OTN, explore a developer's guide to JPA annotations, preview a chapter from Mike Keith's forthcoming EJB3 book, and get other info.