OpenCart - A free shopping cart system. OpenCart is an open source PHP-based online e-commerce solution.s
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
AbanteCart - Free, open source, shopping cart/ e-commerce platform.
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.
Zen Cart - Zen Cart
Braintree - An all-in-one solution to accept, process, and split payments in your mobile app or online - from small business to large enterprise.