WHMCS - WHMCS is an all-in-one client management, billing & support system for online businesses.
Trexle - Payment Platform Supporting One-time and Recurring Billing
BoxBilling - Complete client management, billing & support software.
Paddle - The Paddle Revenue Delivery Platform for B2B SaaS companies powers growth across acquisition, renewals and expansion.
BILLmanager - BILLmanager is an all-in-one web hosting billing software. It provides billing automation, service provisioning, client portal & back office and support center.
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.