40. Square

Founders: Jack Dorsey, Jim McKelvey
Date launched: 2009
Funding: $590 million
Industry disrupted: Mobile payments

Square, the online payment-processing company, is helping small businesses compete with their bigger rivals. The company was started in 2009 by Twitter co-founder Jack Dorsey and blasted onto the scene with its small, square-shaped credit card reader. It plugs into smartphones and makes it easier for small businesses to accept credit card payments. The company charges small businesses a flat fee of 2.75 percent for each payment it processes, with Square claiming it processed more than $30 billion in payments last year in the U.S., Canada and Japan.

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Now the company is expanding its offerings in order to capture more of the small-business market. Last year it introduced Square Capital, a lending program that loans smaller firms from $3,000 to $10,000. So far, the company says it has advanced "tens of millions of dollars" to thousands of small-business owners in the U.S. Square has also raised more than $500 million in financing from Khosla Ventures, Sequoia Capital and Kleiner Perkins Caufield & Byers.