RailsAdmin is a Rails 3 engine that provides an easy-to-use interface for managing your data
The open source error catcher that's Hoptoad API compliant
Copycopter Server is open source. Run it as a web service.
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.
Keep your ActionMailer emails from escaping into the wild during development.
Ruby/Rails interface to the Fluther discussion system