I'm still seeing a lot of confusion about what Anonymous is.
During the last several months, Anon constituencies have been most active in organizing peaceful online protests against services like MasterCard, Visa, and Bank of America (which are attempting to strangle Wikileaks financially) and in supporting the uprisings in Tunisia, Egypt, Libya, and elsewhere in the Middle East. For example, when Mubarek shut down most Egyptians' access to the internet, Anons provided critical help to the Egyptian people by creating and publicizing alternative means of access and other communications.
Part of the problem is that there have been both overt and covert disinformation campaigns going on about the Anons (as well as about Julian Assange and Wikileaks). E.g., during the Anons' DDoS "sit-in" against MasterCard, a rumor was spread that they'd actually hacked the site and had published a bunch of individuals' MasterCard account information. This was a fabrication; in fact, someone had published a list of account numbers that bore no relation to any actual accounts, in a deliberate effort to smear the Anons.
More recently, per The Guardian: "US spy operation that manipulates social media: Military's 'sock puppet' software creates fake online identities to spread pro-American propaganda." And there's no reason to believe that the disinfo efforts uncovered so far are the only ones.
The Anons are at least as diverse as any other large group of people and certainly include pranksters. But the majority seem almost poignantly altruistic; and I believe the results of their major operations have been helpful. Before you accept as true anything about or purporting to be from Anonymous, please check it against a trusted Anonymous news source. For info about the known, "legit" operations, see AnonNews or AnonOps Communications. Other sites have existed but have been taken down; new ones continually arise; again, please use caution re- the source.
Here are a couple of legit Anonymous videos. In the first, they describe themselves and their general purposes. In the second, they describe one of their more recently-launched operations.