HSQLDB is the leading SQL relational database engine written in Java. It has a
JDBC driver and supports a rich subset of ANSI-92 SQL (BNF tree format) plus
SQL 99 and 2003 enhancements. It offers a small (less than 100k in one version
for applets), fast database engine which offers both in-memory and disk-based
tables and supports embedded and server modes. 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.

WWW: http://hsqldb.org/