H2, a free SQL database engine, by the developer of Hypersonic SQL.

Benchmarks (by H2) show it outperforms popular opensource db engines (embedded and client-server), like HSQLDB, Derby, MySQL and PostgreSQL, in many areas.

H2 is free, comes with source code, and you can freely distribute it (but it is NOT open source, yet).

H2 works well with SQuirreL SQL Client, an open source (LGPL) graphical client for JDBC compliant database. It is written in Java, thus platform-independent.

ใครมองหา embedded db engine อยู่ ตัวนี้ก็น่าสนใจนะ H2 (ไม่ใช่การ์ตูนนะ)