Castor
Castor is an open source data binding framework for Javatm?. 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.

Cayenne
Cayenne is a powerful, full-featured Java Object Relational Mapping framework. It is open source and completely free. One of the main Cayenne distinctions is that it comes with cross-platform modeling GUI tools. This places Cayenne in the league of its own, making it a very attractive choice over both closed source commercial products and traditional "edit your own XML" open source solutions.

Hibernate
Hibernate is a powerful, ultra-high performance object/relational persistence and query service for Java. Hibernate lets you develop persistent classes following common Java idiom - including association, inheritance, polymorphism, composition and the Java collections framework.

Ibatis SQL Maps
The SQL Maps framework will help to significantly reduce the amount of Java code that is normally needed to access a relational database. This framework maps JavaBeans to SQL statements using a very simple XML descriptor. Simplicity is the biggest advantage of SQL Maps over other frameworks and object relational mapping tools.

Java Ultra-Lite Persistence
Java Ultra-Lite Persistence is an object-relational mapping tool.

Jaxor
Jaxor is a code-generating OR mapping tool which takes information defined in XML relating to the relational entities to be mapped and generates classes, interfaces and finder objects which can be used from any Java application (including JFC/Swing, J2EE and command-line tools).

JDBCPersistence
JDBCPersistence is yet another OR mapping layer. It differs from its peers in both implementation and API. Using bytecode generation at its core the framework generates classes that implement JDBC Logic which is specific to a table/bean pair.

JDBM
JDBM is a transactional persistence engine for Java. It aims to be for Java what GDBM is for other languages (C/C++, Python, Perl, etc.): a fast, simple persistence engine. You can use it to store a mix of objects and BLOBs, and all updates are done in a transactionally safe manner.

JPOX Java Persistent Objects
JPOX is a full compliant implementation of Java Data Objects (JDO) API. The JDO API is a standard interface-based Java model abstraction of persistence. JPOX is free and released under an OpenSource license, and so the source code is available for download along with the JDO implementation.

ObJectRelationalBridge
ObJectRelationalBridge (OJB) is an Object/Relational mapping tool that allows transparent persistence for Java Objects against relational databases.