Subscription Payments by involve.me - Launch a subscription business in one day
Killbill - Open-Source Subscription Billing and Payment Platform
Chargebee - Chargebee lets you manage subscriptions and payments at scale, handle custom recurring billing scenarios, reduce subscription churn and simplify accounting.
Quiken - All-in-one client and request management platform.
Kable - Instant messaging for business
Stripe Billing - A set of tools for billing and subscriptions