An entrepreneur may have the best product in the world, but if he or she doesn’t accept credit cards, it can be a problem. That’s something St. Louis glass artist Jim McKelvey learned the hard way, and it’s also why he was inspired to create Square. Cofounded with Twitter founder Jack Dorsey, Square lets small business owners begin accepting payment cards immediately without the contracts, expensive hardware, monthly fees or hidden costs that are typically required. Using Square’s intuitive app and a small plastic device that plugs into a mobile phone’s audio input jack, payment cards can be swiped and read anywhere. Customers can have receipts sent to them via email or mobile phone and then access them securely online; they can also use text messaging to authorize every payment in real-time. For those who create a Square account, meanwhile, there’s faster transaction processing and the option of photo verification. Now in limited beta, Square will donate a penny from every transaction to the cause of the user’s choice. It will be widely available in early 2010. The reader devices will likely be free, while the app will cost about USD 1, according to a report in the Los Angeles Times. Similar in many ways to ProcessAway, which we covered earlier this year, Square promises to open up a whole new world of opportunity for sellsumers and minipreneurs. One to get in on early?