WooCommerce is an open source e-commerce plugin for WordPress. It is designed for small to large-sized online merchants using WordPress. Launched on September 27, 2011, the plugin quickly became popular for its simplicity to install and customize and free base product.
Magento is an open-source e-commerce platform written in PHP. The software was originally developed by Varien Inc., a US private company headquartered in Culver City, California, with assistance from volunteers.
Zen Cart is an online store management system. It is PHP-based, using a MySQL database and HTML components. Support is provided for numerous languages and currencies, and it is freely available under the GNU General Public License.
Omnipay is a payment processing library for PHP. It has been designed based on ideas from Active Merchant, plus experience implementing dozens of gateways for CI Merchant. It has a clear and consistent API, is fully unit tested, and even comes with an example application to get you started.
PrestaShop is a free, open source e-commerce solution. The software is published under the Open Software License (OSL). It is written in the PHP programming language with support for the MySQL database management system. PrestaShop is currently used by 250,000 shops worldwide and is available in 60 different languages.