While you can buy a $500 iPad at Amazon.com with a single click, sending even small amounts of cash to a friend or relative is still often a tedious and slow task. In most cases, you wind up doing exactly what you would have in 1957—writing a check and mailing it. The recipient then has to cash it or deposit it in her bank account.

But starting Tuesday, you can just email cash, free of charge, directly from your debit card to anyone else's, regardless of what bank each party uses. There's no login or password to remember...