Growing out of 4chan, they first came to mass public attention in 2008. They attacked Scientology websites & released documents on the church.
Anonymous supported Julian Assange with their ‘Low Orbit Ion Cannon’, this allowed anyone to participate in attacks on websites impeding WikiLeaks.
Anonymous attacks on PayPal cost the company $5.36 million to repair, and also, Anonymous hacked 10 million Playstation accounts, contributing to a $3.2 billion loss for sony.
Hacktivists, with informal links to Anonymous, took down the CIA website for the ‘lulz’. The leader, Sabu, turned FBI informer, revealing secrets on the whole group.