Check out the Java DB demo by Francois Orsini! It shows how a small, mighty database can be embedded in a browser to develop data-rich applications —

To quote the Sun Developer Network:

Java DB is currently included with the Java Enterprise System as the Application Server's developer database and the Portal Server's data store; it is supported natively in NetBeans 5.0; and, it is included as the out-of-the-box development database in the Sun Java Application Server PE 8.2 and the open community Glassfish implementation.