Recurly of the most used payment gateway in US. I have integared it with eyecare application.
I have created some features manually and customise their classic api lib too.
Recurly providing plenty of features and reach third party integration like salesforce, mailchimp etc..
You can get all Recurly feature list from here.
I have used below features of Recurly,
2. Billing Models
3. Usage-Based Billing
6. Manual Payments
8. Adjustments: Charges & Credits