PayPal integration framework and admin interface as well as IPN listener.
This plugins gives you a great framework to use for integrating with PayPal. It stores both live and sandbox API credentials and allows you to switch back and forth easily. All NVP API calls are passed through the framework and default values such as API version, API credentials, and even currency code are added to the request based on settings from the admin panel.
It also has a built in IPN listener that validates messages as coming from PayPal then throws WordPress actions based on messages received. For example it throws "paypal-recurring_payment_profile_cancel" when someone cancels a recurring payment that they had set up with you. It passes along all the info that PayPal sent to the action, so it's simple to create other plugins that use this one.