Dwolla: transactions under $10 are free
For historical reasons, ACH is approximately as fast as airmailing someone money.  When all the stars line up correctly, it is as fast as Fedexing money.  Credit cards are like emailing money (and Paypal was, very explicitly,about emailing money).  Dwolla is based on ACH.There's probably a lot that can be done with cheap, user-friendly ACH in an API, but I don't know if folks really understand the "Your transaction takes 100 hours to settle" bit.