Electronic commerce, commonly known as e-commerce or eCommerce, consists of the buying and selling of products or services over electronic systems such as the Internet and other computer networks. The amount of trade conducted electronically has grown extraordinarily since the spread of the Internet. A wide variety of commerce is conducted in this way, spurring and drawing on innovations in electronic funds transfer, supply chain management, Internet marketing, online transaction processing, electronic data interchange (EDI), inventory management systems, and automated data collection systems. Modern electronic commerce typically uses the World Wide Web at least at some point in the transaction's lifecycle, although it can encompass a wider range of technologies such as e-mail as well.
http://en.wikipedia.org/wiki/Electronic_commerce

A very intuitive Ecommerce system with a fully integrated SEO shopping cart and B2B quote engine.

OpenCart is an open source PHP-based online shopping cart system. A robust e-commerce solution for Internet merchants with the ability to create their own online business and participate in e-commerce at a minimal cost.

Zen Cart truly is the art of e-commerce; a free, user-friendly, open source shopping cart system. The software is being developed by group of like-minded shop owners, programmers, designers, and consultants that think e-commerce could be and...

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...

Magento is a new professional open-source eCommerce solution offering unprecedented flexibility and control.
Magento was designed with the notion that each eCommerce implementation has to be unique since no two businesses are alike. Magento's modular.

PrestaShop is the most reliable and flexible Open-source e-commerce software. Since 2007, PrestaShop has revolutionized the industry by providing features that engage shoppers and increase online sales. The Prestateam consists of over 100 passionate indiv

Shopping Cart Solution, modular templates, SEO optimization, MVC design, Automatic updating multiple currencies, languages, & Countries, Ajax AddToCart, product downloads, product options, multiple tax and weight classes

osCmax is a free, open source and very powerful e-commerce/shopping cart web application. It installs in minutes, and will run with very well even on small shared hosting accounts.

TomatoCart is the best eCommerce platform you can choose to expand your online business into mighty enterprise. Featuring in flexibility,modular design and intuitive user interface,this open source software gets you to your dream of being successful.

Free, simple to use and easy to modify shopping cart script. Minimal requirements: PHP 5.2 +, 2 MB disk space and 5 free minutes to install! Doesn't need MySQL or any changes in Apache.
Advantages: great Google results, very good performance ...

Onxshop is not only great CMS offering integrated in-context editing and full design freedom without the constraints of limiting templates, but it's also stable ecommerce platform used in the current architecture in production environment since 2006.

Axis is a powerful and free ecommerce engine. It has flexible modular structure and outstanding support for theme overriding and custom modules. A lot of great features comes out of the box.

This is a application that allows you to run your own online marketplace or classifieds listing site similar to Craigslist. It was written in PHP and uses MySQL to for data storage.

Booot is a small, performance-optimized CMS with shopping cart functionality currently only available in Russian.

The modified eCommerce Shopsoftware is an OpenSource shopsystem, published under GNU/GPL since the end of 2008.

Integrated and comprehensive: you have high expectations of yourself and your partners. After all, it’s a question of your success in e-commerce. We offer you that little bit more with our comprehensive, integrated solutions.

NeoBill is a web-based Customer Management and Billing solution designed for web hosting providers, particularly hosting resellers. It is developed in PHP, uses MySQL as the backend database, and is licensed under the GPL.

VamCart - Free, Open Source, CakePHP Based Shopping Cart.

Thelia is an e-commerce solution published under LGPL license.
Developed in accordance to web development standards and based on Symfony 2, Thelia meets the following objectives: performance and scalability.

With this new PHP script you will be able to start selling any digital goods, such as MP3s, eBooks, Movies, Software, or any other service you offer. Just within minutes in your own hosting. Just post a new product in your site, share the URL and get paid