Square, the smartphone credit card payment plug-in that allows users to accept card payments through an app, works the same way retailer card devices do. Great for independent merchants, stoop sales even babysitters, Square is available for mobile devices running on Apple's iOS or on the Android platform. It's small — just one inch square — and it's eco-friendly, eliminating paper receipts. Instead, customers receive detailed receipts via e-mail.
Price: Free for the unit, but there is a per-purchase processing fee of 2.75%, plus 15-cents for swiped-in cards and 3.5% plus 15 cents for accounts keyed in at SquareUp