MariaDB
One of the most popular databases on the web with many notable users
MariaDB began as a database server that offers drop-in replacement functionality to MySQL. MariaDB is built by some of the original authors of MySQL, with assistance from the broader community of free and open source software developers. In addition to the core functionality of MySQL, MariaDB offers a rich set of feature enhancements including alternate storage engines, server optimizations, and patches. MariaDB is feature enhanced, community developed and backward compatible with MySQL. MariaDB ships in all major Linux distributions. MariaDB is also backed by the MariaDB Foundation.
MariaDB has grown to also offer a fully synchronous replication solution, MariaDB Galera Cluster and an intelligent proxy called MariaDB MaxScale. MariaDB also has LGPL client libraries for C, Java and ODBC.