Square Register Square, whose CEO is Twitter creator Jack Dorsey, works with a free credit card reader. No contract, monthly fees or merchant account required.
GoPayment You need an Intuit GoPayment Merchant Service account to use this app. If you process less than $1000/month, there is no monthly fee. You receive a free credit card reader. Payment data can be synced with QuickBooks Pro or Premier.
Ring It Up Lite: Legacy Turn your iPhone into a cash register and create sales tickets or invoices in HTML or PDF. Integrates with Credit Card Terminal, Credit Card Reader, Swipe, ProcessAway, or iPay POS Credit Card Terminal.
Credit Card Reader Works with any merchant account. 1.69% + 24c/transaction and free credit card reader.