WooCommerce

WooCommerce – “The most customizable eCommerce platform for building your online business”.  WooCommerce is a free, powerful, and extendable eCommerce plugin that helps you sell anything, beautifully.

This is definitely my favorite eCommerce solution.  You can find lots of free themes for WooCommerce also.  If you want to get an eCommerce site up and running quickly, this is definitely the best choice.