Database currently using by Market leader
Google - Bigtable
Oracle (Relational Database)MySQL (Relational Database
Stack Overflow - SQL Server.
Flickr uses MySQL.
YouTube uses MySQL but they are moving to Google's BigTable.
Myspace uses SQL Server.
Wikipedia uses MySQL.
Hive (Data warehouse for Hadoop, supports tables and a variant of SQL called hiveQL). Used for "simple summarization jobs, business intelligence and machine learning and many other applications"
Cassandra (Multi-dimensional, distributed key-value store). Currently used for Facebook's private messaging.