Jeff Skoll, 39, Chairman & Founder, Skoll Foundation ($4.69 billion)

Jeff Skoll is No.3 on the elite list of richest Americans who have yet to turn 40. He was eBay's first president, creating the business plan that the company still follows. In 1998 he inspired the company to take an active role in philanthropy, pioneering creation of the eBay Foundation through the allocation of pre-IPO shares.

In 1999, he created the Skoll Foundation, which takes an entrepreneurial approach to philanthropy, seeking out and empowering the world's most promising social entrepreneurs in order to effect lasting, positive social change worldwide.

Today, he serves as chairman of the Skoll Foundation and continues to build the foundation. He also serves as a board director for the eBay Foundation and Community Foundation Silicon Valley and is a member of the advisory board of the Stanford Graduate School of Business, among others.

He holds a B.S. in electrical engineering from the University of Toronto and an M.B.A. from the Stanford Graduate School of Business.

Hollywood actor Robbin Williams (L) with Jeff Skoll (R). Photograph: Scott Gries/Getty Images

Also see: The 9 Indian Billionaires