Square Cash
Square Cash is a payment service letting users send money by e-mail. It works by sending an e-mail message to the desired recipient of the money, with a CC to cash@square.com, and the dollar amount to send in the subject line. (The body may have whatever you want or be empty.) If both sender and recipient are already signed up, the cash will be transferred between the registered debit cards of the two people, while if either one is not yet a member they will receive another e-mail message giving them a link to sign up. Once the transfer takes place, both parties are e-mailed about it, allowing them to take action if the transfer was accidental or fraudulent. Fraud does seem to be a risk of the service given how easy it is to forge the from line of an e-mail.
