Kill Bill is the Open-Source Billing & Payment Platform.

Among features:

• 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.

• Tutorials
• User guides (source in the killbill-docs repo)
• Wiki

Kill Bill is released under the Apache license.