Active Merchant: A payment processing library for Ruby / Rails
Active Merchant is a payment processing library for Rails developed by the geniuses behind Rails powered e-commerce system, Shopify. It's under active development with support for different payment processor gateways being added regularly. So far it supports:
They want to integrate with WorldPay and ChronoPay next. The syntax for using Active Merchant and its descendent classes is ridiculously easy, so check it out (code examples viewable there).