&#62; What alternative services are there to PayPal that offer the ability to act only as payment processors that get called via an API?<p>What you're looking for is called a Payment Gateway.  The defacto Ruby library for utilizing them is called Active Merchant (<a href="http://activemerchant.rubyforge.org/" rel="nofollow">http://activemerchant.rubyforge.org/</a>) and it includes ~30 different companies.  If you're looking for a list of all the big ones, that's a good place to start.<p>I've personally used Authorize.net &#38; Paypal before.  Braintree is the company name that gets thrown around in 37signals' circle.