Even the well-publicized efforts of superstars like Bono and Angelina Jolie cannot compare with the potentially globe-changing quest of Warren Buffett and Bill Gates, the two richest men in America. For over a year now, in a new voluntary, ad-hoc association called the Giving Pledge, these self-made mega-billionaires have been urging their peers to donate at least half their fortunes for the common good. They are asking the über-rich in America—as well as around the world—to stand up publicly and commit hundreds of millions, if not tens of billions, of dollars to the charities of their choice.