osCommerce - Open Source Online Shop eCommerce Solution
osCommerce allows store owners to setup, run, and maintain online stores with minimum effort. It could be easily integrated in to the existing site. Administrator could add categories, products, manufacturers, customers, and reviews. Multilingual and Multicurrency support. It accepts numerous online (PayPal, 2CheckOut, Authorize.net, iPayment, ..) and offline (cheque, money orders, offline credit care processing) payments.
osCommerce has built a showcase of over 12,700 online shops that have been voluntarily added to the live shops section, and powers many thousands of more online shops worldwide.