Anonymous

"We are Anonymous. We are Legion. We do not forgive. We do not forget. Expect us."

This the famous quote from anonymous and is there calling card.  Anonymous is a loosely associated hacktivist group. It originated in 2003 on the imageboard 4chan, representing the concept of many online and offline community users simultaneously existing as an anarchic, digitized global brain. It is also generally considered to be a blanket term for members of certain Internet subcultures, a way to refer to the actions of people in an environment where their actual identities are not known. It strongly opposes Internet censorship and surveillance, and has hacked various government websites. It has also targeted major security corporations. Its members can be distinguished in public by the wearing of Guy Fawkes masks. - REF

Below is the anonymous manifesto. Most of there messages, manifesto etc are done though social networking site such as 4chan, youtube and twitter. 

The movements image has been heaviley inspired by the comic book v for vendetta which was written by Alan Moore and illustrated mostly by David Lloyd. V is also heavily inspired by the gunpowder plot. Which is all anti establishment movements. 

This movement has been widely described as a hit and run group who only run on the web, however there supports have been seen at protests and major political events. I would have to say that anonymous is the single most important 21 century movement for there anti censorship stance.