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OpenCart lets you create unlimited categories for your products, support multiple currencies, allow product reviews and ratings, which can be useful if you're selling products from different manufacturers (think Amazon), sell downloadable goods (like ebooks and software), set multiple tax rates, display related products, issue discount coupons, give your customers printable invoices, calculate shipping, etc.

PrestaShop features special deals (where you give a price reduction or gift voucher), cross-selling (eg, to sell accessories), unlimited payment methods, payment by bank wire, the ability to order out-of-stock items, product tagging, return of merchandise, shopping cart recall, sale of services and virtual goods, pick up in store option, product search, an affiliate program, built-in support for PayPal, real-time currency exchange rates, unlimited categories, subcategories, product attribute combinations and specifications, unlimited tax settings, currencies, carriers and destinations, tax by state, country, both state and country, zones or no taxes at all, quantity discounts (digressive pricing), search engine friendly URLs (web addresses), SSL encryption, etc (lots of features). The software is released under the Open Software License (OSL) and requires PHP 5 or later and MySQL 5 or later. (Read their system requirements for more details.) 

Zen Cart is an open source shopping cart software that uses an XHTML template system so that you can customize it easily, supports unlimited extra pages, has multiple shipping options, payment options, tax support, different currency support, and ad-banner controller. It also allows you to set discount coupons, provide gift certificates, give quantity discounts and special prices, etc. The software is released under the GNU General Public License. This is a PHP script. 

osCommerce is an open source online shop solution that uses PHP and MySQL to manage your shopping cart. It allows you to have an unlimited number of products and categories, provides an automatic setup of a variety of e-commerce payment systems, supports secure transactions with SSL, many shipping options including weight, price and destination, taxes (including compounded taxes, etc), database backup and restore, physical products as well as downloads, multi-currency support, etc. 

FreeWebShop is a free PHP shopping cart script that uses a MySQL backend. It supports payment portals like PayPal, etc (custom payment portals can be added as well). It supports product pictures, including auto-generation of thumbnails, IP banning, the ability of customers to track the status of their orders, user-friendly shopping carts where customers can place items into their shopping basket before even logging in), etc. This software is licensed under the GNU General Public License. 

This free PHP shopping cart allows your visitors to add items from any web page on your site, supports the adding of single or multiple items to the cart, multiple payment options, printable purchase orders for customers, etc. You have to keep the site's link and logo on the bottom of every page within the shopping cart package.