You ever wondered what database the big boys use?  Yeah, me too.  Below you will find a table listing what I “think” they are based upon internet research (forums, job listings, etc).  This list might be incorrect and if so I apologize.

Application/Website	Database1	Database2
Twitter	MySQL	Cassandra
YouTube	BigTable	MySQL
MySpace	MS SQL Server
Wikipedia	MySQL
Facebook.com	Hive	Cassandra
LinkedIn	Oracle	MySQL
Digg.com	MySQL	MemcacheDB
StackOverflow.com	MS SQL Server
Microsoft.com	MS SQL Server
Yahoo.com	PostgreSQL
Flickr	MySQL
Google	BigTable
PlentyOfFish.com	MS SQL Server
Friendster	MySQL
ESPN	Oracle
Amazon	Oracle	SimpleDB
Ancestry.com	Hive	MySQL

http://stackoverflow.com/questions/1113381/what-databases-do-the-world-wide-webs-biggest-sites-run-on

http://www.oracle.com/us/technologies/026974.pdf

http://stackoverflow.com/questions/415360/what-types-of-technologies-does-amazon-com-use-internally