Active Merchant is a payment abstraction library sponsored by Shopify and written by Tobias Luetke, Cody Fauser and contributors. It is currently used in production at Shopify, a SaaS company that spawned the initial project and library.
The aim is to create a Rails-esque payment processor library that feels natural to ruby users while extracting as many parts as possible from the user to offer a consistent interface regardless of the gateway selected.
If you are interested in adding support for other gateways please refer to the Contributing document.
Some articles of interest:
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.