SQUARE has been defined as the PayPal for the real world. It allows user to accept micro payments almost everywhere there is a GSM connection:

Square uses a small card scanner which hooks up to a mobile by plugging straight into the audio-in jack. It lets you make physical credit card transaction payments, instantly. Think PayPal but for the physical world. At the moment it works on both the iPhone and Android handsets. […more]

I believe this is an interesting application because it might allow to extend further the concept of virtual economy allowing users to create virtual currencies as forms of payment. Of course, enhancing the range of companies that have access to credit card payments has also social implications for the management of the credit as some people might might be lend to spend more than what they can afford.