Writes Bloomberg: “MySQL and others are starting to eat into the $8.8 billion market for database software dominated by Oracle, International Business Machines Corp. and Microsoft Corp., users said. Yahoo, which uses MySQL to run the Yahoo! Finance Web site, may replace some Oracle databases with MySQL, said Jeremy Zawodny, a computer engineer at the Internet company.”
Among the other free databases is PostgreSQL. I think MySQL is not under GPL so one cannot bundle it commercially for free, while PostgreSQL is.