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Active Merchant is an extraction from the e-commerce software solution Shopify. Shopify's requirements for a simple and unified API to access dozens of different payment gateways with very different internal APIs was the chief principle in designing the library.
Active Merchant has been in production use since June 2006 and is now used in most modern Ruby applications which deal with financial transactions. It was developed for usage in Ruby on Rails web applications and integrates seamlessly as a plugin but it also works excellently as a stand alone library.
It is maintained by the Shopify and Spreedly teams, with much help from an ever-growing set of contributors.
If you are interested in adding support for other gateways please refer to the Contributing document.
A blog post about developing integrations (instead of gateways) for a payment provider: Understanding Active Merchant Integrations
See also http://www.activemerchant.org which has links to the mailing list, etc. This website is not updated regularly but contains a repository of useful information.