6. Charles Koch

• Net Worth: $36 B                                                                   
• Source of Wealth:  diversified, inherited and growing                 
• Citizen: United States
• Age: 78

The world's richest brothers head the nation's second biggest private company with $115 billion in sales. Koch Industries owns firms that are involved in refinery, chemicals and financial trading, among many others. They're well known as megadonors for their conservative and libertarian political agenda and to the arts and medicine, including Lincoln Center and Memorial Sloan-Kettering Cancer Center.