MySQL

MySQL is billed as the world's most popular open source database. It was purchased by Sun in 2008 for $1B. A year later Oracle bought Sun for $7.4B, and as a result acquired MySQL. 

MySQL is licensed under the GNU General Public License, which has led to a lot of confusion over when a paid license is required. You should probably read this Licensing FAQ by MySQL's creator Monty Widenius.