Vitalik Buterin

Vitalik Buterin is the boy genius behind Ethereum, the world's second-most valuable cryptocurrency network behind Bitcoin. The twiggy, Russia-born software developer is arguably as successful an entrepreneur as Evan Spiegel or Brian Chesky. His visionary project has rocketed to a market cap of nearly $30 billion—as high as the private valuations Airbnb and Snapchat achieved—from $1 billion a year ago, when Fortune first placed him on its 40 under 40 list.