The proliferation of credit cards as a payment method preferred by millions of consumers has led to the co-founder of Twitter, Jack Dorsey, creating Square - a credit card reader device that makes any mobile phone a cash register. Retailers can hook up the credit card reader to any device with an audio input jack, including a mobile phone. And consumers never have to worry about carrying cash again (well, almost).
The impetus for Square was an encounter Jim McKelvey (now a member of the Square team) had in February 2009 when he wasn't able to sell his artwork because he couldn't accept a credit card as payment. Now that has changed with Square.