MySQL is a popular choice of database for many developers. It's open source, easy to install, stable, and very fast. However, MySQL comes with several storage engine and you need to decide which one to choose. The two most popular are:
- MyISAM - this is the default storage engine for MySQL prior to version 5.5. I believe this is the fastest storage engine of all, but the drawside is it does not support transactions.
- InnoDB - this is not as fast as MyISAM, but it support transactions. This is the most popular among Java/Grails developers.