WooCommerce - A freely available eCommerce plugin that enables shop facilities on your WordPress website. Functionality enabling extensions & beautiful themes available.
RetailMenot - The RetailMeNot mobile app allows you to find deals on the go for both online shopping and in store shopping.
PrestaShop - Create your online store with PrestaShop's free shopping cart software. Build an ecommerce website for free and start selling online with hundreds of powerful features.
Slickdeals.net - Slickdeals: The Best Deals, Coupons & Discounts on everything
Shopify - Shopify is a powerful ecommerce platform that includes everything you need to create an online store and sell online. Try it free for 14 days.
Honey - Honey is a browser extension that automatically finds and applies coupon codes at checkout with a single click.