ObjectDB - A full featured Object Database for Java, written entirely in Java. JDO compliant. [Free for non-commercial] - http://www.objectdb.com/
ObjectStore - A complex data management and real-time event processing application for Java and C++ applications. - http://www.progress.com/objectstore/index.ssp
FastObjects j2 - A pure Java object database (ODBMS) component embedded into Java applications to provide object persistence. The j2 OODB is compliant with JDO and is designed for use in embedded systems. - http://www.versant.com/en_US/products/fastobjects
Sieben Geisslein - An object store implemented in Java. [Open Source, BSD style] - http://siebeng.sourceforge.net/
Jeevan - 100% pure Java Object Oriented Database, with a simple API and no schema definition language. [Commercial] - http://www.w3apps.com/index.jsp
JYD Object Database - Pure object DBMS written entirely in Java. Multi-user with client-server support and garbage collection. [Commercial] - http://www.jyd.com/
SOD - Alpha-quality object-oriented database providing ODMG-conforming Java and C++ language bindings, for Windows and Linux. [Free for personal use] - http://www.tneumann.de/sod/index.html
db4o - Object database in Java for Java. [Free for non-commercial] - http://www.db4o.com/