Square Cash
Seems to work amazingly although I'm a bit concerned about the security. A friend sent me $1. I get an email from square, link to website where I entered my debit #, expiry and postal. It deposited directly to my debit card (I didnt even know you could do that). The deposit already arrived!"Checking Card Adjustment POS Pin (Credit)  $1.00"So I sent him $1 back (to: my friend, cc: cash@square.com, subject: $1). And it instantly sent it to him. I didn't have to verify my details or anything.I'd feel a lot more comfortable if there was a security blog explaining how they are validating that I indeed sent the email and it wasn't simply spoofed.Edit - I did this from Gmail which I presume authenticates all of the emails via dkim? I'm guessing this won't work as automatic for other providers?Edit2 - Just attempted with another friend and had to verify manually. The automatic-authorization appears to only apply when it's between two previously validated parties.