He was one of four people listed as founders of X.com, which merged with Confinity, which changed its name to PayPal where he was fired as CEO; PayPal was then purchased by EBay, whereupon Musk took a big payday.
Judging by his involvement with Tesla Motors Inc -- he became chairman and CEO by investing 6.5 million into the existing company; a lawsuit brought by an incorporating founder resulted in an agreement whereby Musk and four others can call themselves founders -- makes me wonder what his contribution to X.com actually was.
Whatever else he may be, he's an extraordinarily good investor.