MySQL is the most widely-used open-source database in the world. Many popular open-source applications, from WordPress to SugarCRM to Joomla!, use it. And popular websites like Facebook and Twitter rely on it as well.
The popular database system is offered by MySQL AB, which was purchased by Sun Microsystems in 2008. Sun Microsystems, of course, was just purchased for $7.4bn by database and enterprise software giant Oracle.