From LXF Wiki
The world's most popular relational database management system (RDBMS), available under the GPL from  (http://www.mysql.com/). As of its current stable release, v4.0, MySQL lacks many of the more popular features of more powerful databases, including sub-queries, triggers, and stored procedures, however these are planned for v4.1 and v5.0.
MySQL is known for being incredibly quick, and this opinion is justified - it flies.
MySQL is the "M" in LAMP - the de facto industry-standard free stack for hosting web services.