Percona : MySQL open source fork with performance improvements
Percona is a company that features their compilation of MySQL with emphasis on performance. That MySQL software is best known as Percona Server for MySQL. They have downloads for Percona Server for MySQL (5.7, 5.6, 5.5, 5.1), Percona XtraDB Cluster, Percona Server for MongoDB and tools for backup, monitoring, online schema changes and more.
One of their most notable enhancements is the storage engine XtraDB (complete drop-in replacement for InnoDB) which has been around since December 2008.
Percona also has a well-known blog, previously known as the 'MySQL Performance Blog', now called the 'Database Performance Blog'.