The new version of HSQLDB, the leading SQL database engine written in Java, has just been released. HSQLDB is a relational database engine written in Java, with a JDBC driver, supporting a large subset of ANSI-92 SQL. A small, fast engine with both in memory and disk based tables. This product is the continuation of HypersonicSQL. Active since 2001.
See http://hsqldb.org for the new features.