Active Merchant is a simple payment abstraction library used in and sponsored by Shopify. It is written by Tobias Luetke, Cody Fauser, and contributors. The aim of the project is to feel natural to Ruby users and to abstract as many parts as possible away from the user to offer a consistent interface across all supported gateways.
Addressable is a replacement for the URI implementation that is part of Ruby's standard library. It more closely conforms to RFC 3986, RFC 3987, and RFC 6570 (level 4), additionally providing support for IRIs and URI templates.
Authority helps you authorize actions in your Rails app. It's ORM-neutral and has very little fancy syntax; just group your models under one or more Authorizer classes and write plain Ruby methods on them.