40 under 40

25. Andrew Mason

In 2008, Mason launched Groupon, a website that offers discounts on local goods and services if enough people sign up (the name is a mashup of "group" and "coupon"). It has since amassed 18 million subscribers, attracted $175 million in funding, and spread to 29 countries. It has also spawned hundreds of clones, but none as big as the original: Its most recent round of investment valued the company at $1 billion.

Leisure pursuit: Mason has his own archery range.

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Last updated October 14 2010: 1:25 PM ET