WooCommerce is an eCommerce plugin for WordPress, developed by WooThemes (recently acquired by Automattic). Like WordPress, it is designed to be an extendable, adaptable, open-sourced platform. WooCommerce allows merchants to sell physical products, downloadables, or services. The free core includes: Storefront, a free WordPress theme designed for eCommerce, 5 pre-installed payment gateways (including PayPal and Direct Bank Transfer), coupons, shipping and cart calculators, product listing management and customer account registration, among other features. Many many extensions are available, such as WooCommerce Bookings, for e-merchants selling appointments, services, or rentals.
According to WooThemes (per Builtwith.com), WooCommerce has > 9,524,908 downloads and powers > 30% of online stores.