Elon Musk, co-founder of PayPal and founder of Tesla Motors, joined the list as the world's 634th richest person as the value of stock in his electric car company rose. The widow of Apple's Steve Jobs, Laurene Powell Jobs, took his spot on the list. But one of the most interesting additions was Sara Blakely, the founder of women's slimming undergarments company Spanx, who joined the list this year. At age 41, Blakely is the world's youngest self-made female billionaire.