15. Niklas Zennstrom and Janus Friis, Founders, Skype and KaZaA These two just can't stop themselves. First they built the peer-to-peer file-sharing network KaZaA; then they followed that endeavor up by building the amazingly popular VoIP software Skype. After selling Skype, the duo has gone back to the drawing board to produce Joost, a P2P video distribution service that is currently in beta form. Will Zennstrom and Friis pull off a trifecta of killer apps?