Created by the New Zealand-based Instinct Company, WP e-Commerce is an easy and elegant way to integrate e-commerce capabilities into WordPress. The plugin is free (released under the GNU General Public License), though some additional modules are available as part of a paid upgrade.
WP e-Commerce is the product of over four years of development and testing. With over 200,000 downloads to its fame (as of this writing), and featured as one of the most popular plugins for WordPress, WP e-Commerce is the most tried-and-true way to build an online store with WordPress.
It's an ideal solution for selling physical goods, services, and digital products online.
Downloads are available at http://getshopped.org.