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InvoiceNeko - An open-source and free invoicing system

Recurly - Subscription billing and recurring billing management. Recurly offers enterprise-class subscription billing for thousands of companies worldwide.

Invoice Template - Create beautiful looking invoices in seconds

Zuora - Zuora creates cloud-based software on a subscription basis that enables any company in any industry to successfully launch, manage, and transform into a subscription business.

Invoice.app for Mac - Invoices made easy