Thin identity -- the missing web service

  I wish there were a web service that had a simple XML and/or JSON interface that made it easy for a user to establish a username and password.
  Such a service, if it had reasonable terms of service, could clean up among developers who want to invest in a net outside of Twitter, Facebook and Apple.
  Why doesn't Amazon have a simple authentication service? This person says he or she is bullmancuso. Great, is this his password? Yes or no. That's all we need.
  How about Visa or Mastercard. A bank?
  I suspect we don't have it because of the confusion about OAuth. Shame cause it's desperately needed.