MariaDB

MariaDB: Free open-source database server with a pretty hefty list of compatible clients and features.

MariaDB is a great open-source database server that was designed by the original developers of MySQL and is being used by Wikipedia, Google, Facebook, etc.

It comes with the most recent edition of the database server that includes all MySQL 5.5 features, like Interface, APIs, libraries, and commands, while also comes with support for GIS and JSON, XtraDB storage engine that replaces InnoDB, in addition to Aria, which is a newer storage engine, that will become a crash-safe alternative to MyISAM, and the standard transactional and non-transactional storage engine for the server.

The database server comes with a pretty hefty list of compatible clients (e.g. Database Workbench, DBEdit, HeidiSQL, Navicat, phpMyAdmin and SQLyog), and also, the official support will be provided by Drupal, ERP5, Kajona, MediaWiki, and Moodle.

All in all, MariaDB is a great free and open-source database server that was designed to provide everyone with a lot of compatible features and clients.