Stand-Alone Carts

PrestaShop – A free, open-source, scalable shopping cart with an intuitive interface.

OpenCart – A free, PHP-based, open-source shopping cart that includes support for product reviews, automatic image resizing, and downloadable products.

Magento – An e-commerce platform that includes both free community versions and paid enterprise versions.

CubeCart – A shopping cart with two versions available, one of which is free.

Zen Cart – A free open-source shopping cart with free add-ons available.

osCommerce – A free and open source shopping cart that powers more than 225,000 online stores around the world.

Digistore – A free, open source shopping cart built on osCommerce.

StoreSprite – A free and powerful PHP/MySQL shopping cart that’s easy to use.

BossCart – A PHP-based shopping cart with free and premium versions available.

FatFreeCart – A copy-and-paste shopping cart solution that works with PayPal or Google Checkout and doesn’t require you to sign up for an account or install anything.

Plugins

WordPress e-Commerce – A free, open source plugin for WP with free and paid add-ons.

Shopp – An incredibly easy to use, premium shopping cart for WP.

Ubercart – A shopping cart plugin for Drupal 5 or 6.

Drupal e-Commerce – An e-commerce plugin for Drupal that works with PayPal, Authorize.net, and other payment systems.

VirtueMart – A free shopping cart plugin for Joomla.

Seber Cart – A small, easy to use shopping cart plugin for Joomla.

Joomla jQuery Cart – jCart is an easy-to-install and customize, free Ajax shopping cart.