NoSQL: What is the “best” solution for storing high volumes of structured data?
My answer to NoSQL: What is the “best” solution for storing high volumes of structured data? on Quora
On the right setup, PostgreSQL can handle petabytes. There are also commercial vendors such as Greenplum that offer data warehouse solutions built on a modified version of PostgreSQL.
You should also take a look at Hadoop and Hive. Hive lets you use a language based heavily on SQL to construct map reduce queries which can then run against a Hadoop cluster.