The world's second-richest man is worth $65 billion -- and that's after giving away more than $28 billion. Gates' post-Microsoft mission includes eliminating many infectious and deadly diseases: By his own estimates, that could translate into 8 million lives saved by 2020. But the quintessential activist billionaire doesn't stop there: Gates continues to persuade his peers to sign the "Giving Pledge," promising to give away half their wealth or more. 2012 HIGHLIGHT: Twenty-three more tycoons signed the Giving Pledge this year, bringing the total to 92.