Bill Gates, Melinda Gates, and Warren Buffett are asking the nation's billionaires to pledge to give at least half their net worth to charity, in their lifetimes or at death. If their campaign succeeds, it could change the face of philanthropy.

Buffett, Gates, and Gates -- who have a combined net worth of around $100 billion -- have already committed most of their money to charity. Click on the photo for more pictures.