The weapon of choice for the hackers is the same as it was when the group took down websites belonging to MasterCard, PayPal, and Visa in December 2010, because the payment processors declined to transfer money to WikiLeaks. It's a small program called Low Orbit Ion Cannon that sends out a flood of fake traffic to a selected website, overloading it if it doesn't have enough capacity.
Anonymous is apparently very interested in targeting countries that are very unstable; currently the group is focusing on antigovernment protests that have roiled the Arab world. Last month, the group shut down websites belonging to the Tunisian government and stock exchange in support of the uprising that forced the country's dictator, Zine el-Abidine Ben Ali, to flee.
A revolution is currently taking place in Egypt, and Yemen is apparently next. Egyptians only recently had their Internet restored, after a five-day blackout. Yemenis, on the other hand, will probably not notice Anonymous' work in their country, given that only about 5 percent of them have access to the Internet.