WHMCS - WHMCS is an all-in-one client management, billing & support system for online businesses.
PayPal - PayPal is the faster, safer way to pay online without sharing financial details, send and receive money or accept credit and debit cards as a seller
BoxBilling - Complete client management, billing & support software.
Stripe - Online payment processing for internet businesses. Stripe is a suite of payment APIs that powers commerce for online businesses of all sizes. Use Stripe’s payment platform to accept and process payments online for easy-to-use commerce solutions.
HostBill - HostBill is a powerful and flexible, all-in-one automation, billing, client management and support platform for online businesses.
Braintree - An all-in-one solution to accept, process, and split payments in your mobile app or online - from small business to large enterprise.