Subscription management and processing (TrustCommerce, BrainTree, etc.)
Billing for rack apps
Ruby lib for using the FastSpring (SaaSy) subscription management API
SubscriptionFu helps with building services which have paid subscriptions. It includes the models to store subscription status, and provides integration with PayPal for paid subscriptions.
This was purpose built to extend models with subscriptions that were controlled via SalesForce