FirstSQL/J
A pure Java fully SQL/92 compliant ORDBMS. [Commercial]
url: www.firstsql.com/
IBM Cloudscape
Pure java database management system for distributed, embedded, mobile computing applications. [Commercial]
url: www-3.ibm.com/software/data/cloudscape/
Inprise JDataStore
A Pure Java, Object Relational Database Management System for use in Web, mobile or embedded database applications. [Commercial]
url: www.borland.com/jdatastore/
JGrinder
Initially began as a solution for Object/Relational persistence. Supporting this involved the creation of many utility classes and tools that could be used to accelerate any application development effort. [Open source, LGPL]
url: jgrinder.sourceforge.net/
Sav Z
Web ORDBMS with Zigzag language for object, relational and XML data manipulation. [Free for non-commercial]
url: savtechno.com/
StoredObjects
Cross-platform object relational database for Java. For beginners it is a complete development environment for writing database applications in Java without reading a manual. For experts it is a tool for developing distributed client/server groupware in Java (TM). [Open Source, GPL]
url: storedobjects.sourceforge.net/