Megaupload was a company/website based in Hong Kong that ran a multitude of widely used services including: Megaupload, MegaVideo, MegaPix, MegaLive, and MegaBox. With 81,000,000 unique visitors and 490,000,000 page views, Megaupload was a very widely site, particularly by those who are considered to be "pirates".

Yesterday (January 19, 2012), the United States Justice Department shut down Megaupload without due process while investigating copyright infringement. Millions of dollars in assets were seized. Megaupload's founder, Kim Dotcom, as well as four of Megaupload's seven executive were arested. Extradition proceedings have already begun and are projected to last more than a year. "Dotcom is accused of heading up a criminal venture that illegally cheated copyright holders out of $500m in revenue over a five-year timeframe" (theguardian).

As a result of the shutdown, hacker group Anonymous launched a retaliatory attack on the US Justice Department website. And multiple other versions of Megaupload appeared across the web in "high tech game of whack-a-mole" according to ABC News. Anonymous tweeted today:

"US Department of Justice http://justive.gov TANGODOWN (yeah again and again...) #ALLYOURBASEAREBELONGTOUS #Anonymous #cocks,"

The US Justice Department's site, justice.gov, has been intermittently accessible today, and according to the Department of Justice, their server was experiencing "a significant increase in activity, resulting in a degradation in service".

Anonymous also targeted websites belonging to the FBI, RIAA, MPAA, and Universal Music.

It is most speculated that Anonymous most likely used a low orbit ion cannon (LOIC) to DDoS the websites. And many sites also deployed a tool to DDoS a specific target automatically upon loading.

"We've seen many links posted on Twitter, and no doubt elsewhere on the Internet, pointing to a page on the pastehtml.com website," Sophos analyst Graham Cluley said in a blog post. "If you visit the webpage, and do not have JavaScript disabled, you will instantly, without user interaction, begin to flood a website of Anonymous's choice with unwanted traffic, helping to perpetuate a DDoS attack."

Sources:
PCMag.com
theguardian
Wikipedia