JPOX - http://www.jpox.org/
JPOX is a free and fully compliant implementation of the JDO 1.0 and 2.0 specifications, providing transparent persistence of Java objects. It supports persistence to all of the major RDBMS on the market today, supporting all of the main Object-Relational Mapping (ORM) patterns demanded by today's applications, allows querying using either JDOQL or SQL, and comes with its own byte-code enhancer. JPOX 1.0 implements the JDO 1 specification and passes the JDO 1 TCK. JPOX 1.1 extends the JPOX 1.0 capabilities to implement the JDO 2 specification and pass the JDO 2 TCK. JPOX 1.1.0-final is the JDO 2 Reference Implementation. JPOX will be updated in the future to also implement the JPA specification of Java persistence (part of EJB3). JPOX is available under the Open Source Apache 2 license.

Hibernate - http://www.hibernate.org/
Hibernate is a powerful, ultra-high performance object/relational persistence and query service for Java. Hibernate lets you develop persistent objects following common Java idiom - including association, inheritance, polymorphism, composition and the Java collections framework. Extremely fine-grained, richly typed object models are possible. The Hibernate Query Language, designed as a "minimal" object-oriented extension to SQL, provides an elegant bridge between the object and relational worlds. Hibernate is now the most popular ORM solution for Java.

OJB - http://db.apache.org/ojb/
ObJectRelationalBridge (OJB) is an Object/Relational mapping tool that allows transparent persistence for Java Objects against relational databases.

Torque - http://db.apache.org/torque/
Torque is a persistence layer. Torque includes a generator to generate all the database resources required by your application and includes a runtime environment to run the generated classes.

Castor - http://castor.exolab.org/
Castor is an open source data binding framework for Java. It's basically the shortest path between Java objects, XML documents and SQL tables. Castor provides Java to XML binding, Java to SQL persistence, and then some more.

JDO - http://java.sun.com/products/jdo/
The Java Data Objects (JDO) API is a standard interface-based Java model abstraction of persistence, developed as Java Specification Request 12 (JSR 12) under the auspices of the Java Community Process. If you are an application programmer, you can use JDO technology to directly store your Java domain model instances into the persistent store.

TJDO - http://tjdo.sourceforge.net/
TriActive JDO (TJDO) is an open source implementation of Sun's JDO specification (JSR 12), designed to support transparent persistence using any JDBC-compliant database. TJDO has been deployed and running successfully in many commercial installations since 2001.