Occupation: Silicon Valley kingmaker
In the 1990s, Andreessen co-created Mosaic, the first popular Web browser, and then the even more popular Netscape Navigator. That's not why he's influential today though. His venture-capital firm, Andreessen Horowitz, is just three years old but has already raised a stunning $2.7 billion and invested it in companies such as Airbnb, Facebook, Groupon, Skype and Twitter. He serves on the boards of eBay, Facebook and HP. At 40, Andreessen is already one of Silicon Valley's most respected elder statesmen.