Magento - a feature-rich e-commerce platform with a large developer community

WooCommerce - a popular WordPress plugin that adds e-commerce functionality to a website

OpenCart - a lightweight e-commerce platform that is easy to set up and use

PrestaShop - a flexible e-commerce platform with a variety of features and integrations

osCommerce - a legacy e-commerce platform that is still in use by many small businesses

Zen Cart - another legacy e-commerce platform that is still actively maintained

TomatoCart - a relatively new open-source e-commerce platform based on the osCommerce codebase

Ubercart - an e-commerce solution for Drupal websites

Spree Commerce - an open-source e-commerce platform built with Ruby on Rails

VirtueMart - a Joomla plugin that adds e-commerce functionality to a website