hackforums.net
In June 2011, the hacktivist group known as "LulzSec" leaked one final large data breach they titled "50 days of lulz". The compromised data came from sources such as AT&T, Battlefield Heroes and the hackforums.net website.
A security/data breach is the intentional or unintentional security incident in which information was accessed without authorization. Release of secure or private/confidential information to public can hurt businesses and consumers in a many of ways.
Other reference: security compromise, data leak, information disclosure, information leakage, data spill.
List of all known breaches.