Ripple

Ripple is a monetary system that makes simple obligations between friends as useful for making payments as regular money.

You can create a profile on the system and indicate who you know and how much you trust them by connecting to people by email address and giving them credit limits. Then whenever you want to make a payment to another Ripple user using only friendly obligations, the system finds a chain of intermediaries connecting you to the person you want to pay, and records the payment in each intermediary's account all the way down the chain.