Square, the very publicized and funded iPhone app that lets anybody take credit cards payments is finally available on the App Store. It was already there since about a month for the iPad, it has however just turned universal.
The Square app is completely free, including the "square" card reader which they'll send you once your account is activated. The app is also very simple and streamlined, as you would expect it to be, and you should be ready to take payments in a matter of minutes. The processing fees are the following:
2.75% + 15¢ for swiped transactions 3.5% + 15¢ for keyed-in transactions
Whether you're a merchant, or your friends owe you a lot of money, this is just what you need. Here is a demo (you can ignore the end about the private beta):