Hibernate is a popular open source Object-Relational mapping tool. This article by Mario Aquino provides a quick introduction to get started with Hibernate.

A lightweight persistence framework for Java (JDK5 or later). [Open source, GPL]

A framework that provides transparent persistence of Java objects for JDBC-compliant RDBMS. Implements ODMG 3.0, JDO and uses XML-based O/R Mapping. [Open Source, LGPL]

Provides the following features: build generated client side data objects structurally mirroring server side business objects; pluggable persistence; pluggable binding between client and server; pluggable query mechanism; zero-configuration externaliz...

Object/Relational mapping tool, with customized mappings for each major EJB server (including JBoss). [Commercial]

Lightweight and flexible solution for storing and retrieving java objects into and from relational databases, without using SQL. [Open source, LGPL]

An Object-Relational mapping tool. Generate persistent enable Java source from Class Diagram or ERD. A diagram editor is provided. It can generate code for client applications and server side applications. [Commercial]

An Object-Relational Mapping Framework with a GUI Editor that lets you automatically create Java objects that map to relational databases. [Open Source, Mozilla-like]

ORM project from Eclipse (Oracle, Sun). Provides ORM, JPA, OXM, JAXB, SDO, WebServices, EIS. JPA 2.0 reference implementation, JPA provider of Oracle WebLogic, Sun Glassfish. [Open source, EPL]

A persistence framework with forms automation. It support on-the-fly SQL table creation/structure modification base on definition in Java class, automated form creation and automated form result parsing. [Freeware]