ObjectDB is a powerful yet compact Object Database (ODBMS) for Java,
written entirely in Java, and compliant with the Java Data Objects
(JDO) standard. ObjectDB is designed to handle efficiently databases
of various sizes, ranged from a few KBs to hundreds of GBs. Among its
features are small footprint, high performance, rapid development,
easy deployment and flexible architecture (1-Tier, 2-Tier or N-Tier).

ObjectDB is the easiest way to Java Data Objects (JDO), the new
revolutionary database programming technology by Sun Microsystems.
Instead of using relational databases with or without mapping tools,
tables, records, fields, SQL, JDBC and drivers, you can download
ObjectDB and start writing more effective database code using Java
classes and objects!

ObjectDB Home:
 http://www.objectdb.com

A Free Database Edition is available for non commercial use:
 http://www.objectdb.com/database/jdo/free-download/