7. Square

Square

Estimated Value: $3.2 billion
Last Year's Rank / Valuation: #14 / $1.6 billion
Business: Accept credit card payments anywhere with your iPhone, iPad or Android phone
Location: San Francisco, California

More Info: About Square
CEO: Jack Dorsey

Investors:  In 2009, Khosla Ventures invested $10 million in Square. In January, Square raised $27.5 million from Sequoia Capital, Khosla Ventures, and Jeremy Stoppelman. In June, Square raised a massive $100 million round led by Kleiner Perkins Caufield & Byers and Tiger Global Management. Later in 2011, Square raised $3M from Richard Branson. In August, Square raised $25M from Starbucks and CrunchFund.

Analysis: While there are a lot of mobile payment solutions, Square is really attacking the cash register space and it's on-boarding small businesses quickly.
Last month Square authorized $200 million in new shares at a valuation of $3.2 billion. And the deal with Starbucks could double its business quickly.
Now, Square is processing U.S. credit and debit transactions for Starbucks which is huge: Starbucks' annual sales in fiscal 2011 were almost $12 billion, about two-thirds of that in the United States.