Kill Bill is the Open-Source Billing & Payment Platform.
- Subscription engine, with plans management (trial, upgrade, downgrade, etc.), support of add-ons, bundles with multiple subscriptions
- Invoicing engine, supporting different billing alignments, recurring and one-time charges, international tax, metered billing
- Payment state machine, with payment routing capabilities, supporting dozen of gateways
- Plugin architecture, which allows further customization with your own business logic, in Java or Ruby
You can find more information on killbill.io.
Kill Bill is released under the Apache license.