MySQL is probably the worlds most popular database. We say that because it is free, easy to use, and is closely tied to PHP and many open source applications. We also like PostgreSQL because it is very easy to install on Linux or Windows. Oracle, IBM DB2, or Microsoft SQL may be more prevalent in large corporations, but small companies who can't afford one of the dominant license based products use MySQL and PostgreSQL more often than not.
"The world's most popular open source database" - MYSQL.COM
We are vendor neutral, and will use whatever database best meets the needs of our clients; however, by default, many open source projects use MySQL.