Derby is a Java RDBMS undergoing incubation at Apache Software Foundation. It is a fully functioned standards based JDBC & SQL RDBMS with tables, indexes, views, triggers, sub-queries, procedures, functions, transactions, isolation levels, encryption, etc. Derby is the open sourcing of the IBM Cloudscape technology.

hsqldb is a relational database engine written in Java, with a JDBC driver, supporting a rich subset of ANSI-92 SQL (BNF tree format). It offers a small (less than 160k), fast database engine which offers both in memory and disk based tables. Embedded and server modes are available. Additionally, it includes tools such as a minimal web server, in-memory query and management tools (can be run as applets) and a number of demonstration examples.

The Ozone Database Project is a open initiative for the creation of an open source, Java based, object-oriented database management system. The ozone project aims to evolve a database system that allows developers to build pure object-oriented, pure Java database applications. Just program your Java objects and let them run in a transactional database environment.