HSQLdb is a relational database engine written in JavaTM , with a JDBC
driver, supporting a subset of ANSI-92 SQL. It offers a small (about
100k), 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 or servlets, too) and a number
of demonstration examples.

Downloaded code should be regarded as being of production quality. The
product is currently being used as a database and persistence engine in
many Open Source Software projects and even in commercial projects and
products! In it's current version it is extremely stable and reliable.
It is best known for its small size, ability to execute completely in
memory and its speed. Yet it is a completely functional relational
database management system that is completely free under the Modified
BSD License. Yes, that's right, completely free of cost or
restrictions!