He's better known to me and my online pals as "tylerknowsthis", a reference to Tyler Durden and his philosophy of destroying the capitalist system to "free the people." Say what you will about his ideals, his methods and actions are beyond retarded.
Here he is at Defcon in 2004 talking about how they need more "footsoldiers" to "fuck shit up in the streets" - to the point that Priest has to come on stage and denounce violent acts or acts that hurt people. http://video.google.com/videoplay?docid=1269112265902193941 In general he defends the use of violence as the last act of a person who is desperate to defend freedoms for people who didn't ask to be helped. His website HackThisSite is a sort of propaganda and training tool used to entice young black hats to join his cause.
You can find a list of his previous run-ins with the law on his wikiepdia page: http://en.wikipedia.org/wiki/Jeremy_Hammond (My favorite is where he attacked a 70-year-old holocaust denier that was having dinner at a restaurant.... what productive direct action!)
He claims he steals his power, water and internet access and at times squats abandoned buildings and eats "freegan" so he isn't helping the capitalist system flourish. At the same time he kept a part-time computer programming job to make spare cash. So he can keep fighting the good fight against capitalism.
I think he may have still been on probation during the events of the Stratfor hack, so he may be royally fucked by the prosecution unless he too snitches - something he has repeatedly said is the worst thing any good hacktivist can do.
He's one of the longest-running jokes my online friends and I have. His rants against "the system" and hypocritical actions which seem to have little purpose serve to foster flame wars and is frequently banned when people get tired of his shit. He then comes back and threatens to "curbstomp" or "shiv" anyone who disliked or banned him. Basically, nobody but the LulzSec freaks like this guy.
Yet again the same people who try to get away with petty online crime get caught due to negligence, bragging and misplaced trust in other criminals. If only they'd learn that trusting a criminal is probably not a good idea they might not be arrested right now.
edit: In case anyone wants to verify this account (in a WikiLeaks-style full transparency way), here is a brief dump of a public chatroom on a public irc server of his comments. I don't have the entire log, just his comments. http://pastebin.mozilla.org/?dl=1506078 http://tinypaste.com/a104418f (it's around 1.8MB)