CrateDB!

MySQL Community Edition is a freely downloadable version of the world's most popular open source database that is supported by an active community of open source developers and enthusiasts.

MariaDB is a backward compatible, drop-in replacement of the MySQL® Database Server. It includes all major open source storage engines.

MongoDB (from "humongous") is a scalable, high-performance, open source NoSQL database. The database is document-oriented so it manages collections of JSON-like documents. Many applications can thus model data in a more natural way, as data can be nested in complex hierarchies and still be query-able and indexable.

Percona Server is an enhanced, drop-in MySQL® replacement which has been downloaded more than 1,400,000 times. Percona Server is based on the Percona XtraDB Storage Engine, an enhanced version of the InnoDB storage engine for MySQL and MariaDB.