MySQL is the most popular open source database in the world. Companies like Yahoo!, NASA and the Associated Press, lookig for an affordable database solution, have embraced MySQL and pushed it to the limit. MySQL AB, the company responsible for developing the MySQL database, has responded by incorporating new and exciting features without sacraficing the speed and flexibility that has made MySQL so popular. By version 5 MySQL will support advanced databse features like stored procedures.
We have been using MySQL for five years and reccomend it for all web applications. Although we have not pushed it to the same limits as Yahoo! (Yahoo! has some tables with 260,000,000 records and one client reported 60,000 tables with 5,000,000,000 rows) we have built applications containing tables with 3,000,000 rows and consuming 600 meg. We have a deep knowledge regarding how to install, correctly configure and write efficient code for MySQL.