At Astrient we use MySQL for all of our web applications. As an alternative,
we have found most developers to consider the PostgreSQL database. Which is why I was very surprised when I stumbled upon the H2 database. From the details presented on the H2 website, the database engine not only performs well but packs a comprehensive set of features and can be embedded in applications. I would recommend anyone looking for an alternative to MySQL and PostgreSQL to visit:

http://www.h2database.com/html/main.html