PayPygg is a service available as a mobile-friendly web app, and it also lets you transfer money to friends through the Twitter interface … if you are Australian that is. For now, the service is available only to people with an Australian bank account, but prospects are that it will soon expand into other countries.
To send money, simply tweet @paypigg pay @[id of your friend] $[amount]
PayPygg payments through Twitter are free. If your friend does not have an account, the payment is kept in suspension and an invitation is sent to the recipient asking him to open an account.
More information on how it works here.