CubeCart is an increasingly popular shopping cart software choice. The cart is based on PHP and mySQL and is praised for its simplicity and ease of use right out of the box. Many hosting providers provide an auto install option using Fantastico in the cPanel. CubeCart is free to use provided you keep their copyright in place. However it is not open source software meaning you can make modifications for your personal use but it can not be freely redistributed. If you wish to remove the copyright notice simply install the software and purchase a license key.
The software is not overly feature rich but provides the most common desired features and boasts a powerful template system allowing easy "skinning" of your site while keeping the design separate from the code. Many addons are available for any additional features you might need and the cost and installation is not usually significant.
More information available on CubeCart official web site.