WooCommerce is a plugin by woothemes that can transform your WordPress website into a thorough-bred online store.
Read more about the features and benefits of WooCommerce at http://www.woothemes.com/woocommerce/
Checkout and contribute to the source on GitHub at http://github.com/woothemes/woocommerce/
Plugin download: http://wordpress.org/extend/plugins/woocommerce/