Active Merchant is a payment abstraction library sponsored by Shopify and written by Tobias Luetke, Cody Fauser and contributors. Its currently in production use in Shopify, the project which spawned the library.

The aim is to create a Rails-esque payment processor library which feels natural to ruby users and extracts as many parts as possible away from the user to offer a consistent interface no matter which gateway you end up using.

If you are interested in adding support for other gateways please refer to the Contributing document.

Some articles of interest:

• Contributing
• GatewayFeatureMatrix
• GatewaysVsIntegrations
• GettingStarted

For a great introduction to Active Merchant check out the PeepCode ActiveMerchant PDF

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.