OpenCart - A free shopping cart system. OpenCart is an open source PHP-based online e-commerce solution.s
WooCommerce - A freely available eCommerce plugin that enables shop facilities on your WordPress website. Functionality enabling extensions & beautiful themes available.
AbanteCart - Free, open source, shopping cart/ e-commerce platform.
PrestaShop - Create your online store with PrestaShop's free shopping cart software. Build an ecommerce website for free and start selling online with hundreds of powerful features.
Zen Cart - Zen Cart
Thirty Bees - Open e-commerce platform designed to work out of the box.