MySQL - The world's most popular open source database.

MySQL is a well-known relational database manager used in a wide variety of systems, including Twitter, Wikipedia, Facebook, Google, Wordpress, and countless more websites and other applications. Offering a wide range of features and highly scalable performance, MySQL is a good choice for any situation requiring a database.

Features

Cross-platform support.
Stored procedures, allowing you to centralize SQL calls.
Triggers, allowing automatic responses to different actions.
Cursors, allowing easy traversal over database records.
True Varchar support.
X/Open XA distributed transaction processing support (with two phase commit as a part) using Oracles InnoDB engine.
Independent storage engines.
SSL support.
Query caching.
Sub SELECTS.
Replication support with one master per slave, many slaves per master. No automatic support for multiple masters per slave.
Full text indexing and searching with MyISAM engine.
Embedded database library.
Partial Unicode support.
ACID compliance when using transaction capable storage engines.
Partitioned tables with pruning of partitions in optimizer.
Shared-nothing clustering through MySQL Cluster.
Hot backup under certain conditions.
Multiple storage engines, allowing developers to choose one that is most effective for each table in the application.
Commit grouping, gathering multiple transactions together to increase the number of transactions per second.