Here's one reason they've been able to muster so much firepower: Anonymous members are distributing a link that ropes internet users into an illegal DDoS attack against these websites simply by clicking it. The link is being shared widely on Twitter and in Anonymous chat rooms, often with no context except that it relates to Operation Megaload. I clicked it a few minutes ago because it was being spammed in an Anonymous chatroom and found myself instantly DDoSing Universalmusic.com, my computer rapidly pinging the page with no way to stop except quickly closing the window.
http://gawker.com/5877707/