osCommerce is the leading Open Source online shop e-commerce solution that is available for free under the GNU General Public License. It features a rich set of out-of-the-box online shopping cart functionality that allows store owners to setup, run, and

phpShop is a free PHP-based e-commerce application developed by Edikon. phpShop allows you to setup an online store and manage all aspects of your online business by using a web browser.

BakeSale is a shopping cart built using the CakePHP framework, web standards, and best practices. It is accessible, search engine optimized, easy to extend and style, and open source!

AFCommerce is a complete online store with both a store front and administration area directly Out-Of-The-Box, which can be easily installed, configured, and maintained over a web-based interface.

The freebert.com Free PHP Shopping Cart is a free PHP package that allows users to have their own, customizable, built in shopping cart. The cart is designed to help your customers easily add and manage items in their shopping cart, and to give them a choice in payment.

Magento is a Zend Framework powered open-source eCommerce solution that features modular architecture and unprecedented flexibility.

Not being happy about current e-commerce solutions, a well-respected DevNet Junkie proposed to rebuild OsCommerce from scratc: Devnet Store.

AlegroCart is an open source e-commerce solution based on the final version of Opencart .79 with many modifications and enhancements. MVC framework, modular design, supports custom add-ons.