AgoraCart is one the most popular shopping cart sofware options available. Many web hosting solutions come with this software package already included as an automatic installation option using cPanel. AgoraCart is Perl/MySQL based shopping system that provides includes "Free" Open Source ecommerce shopping cart software with a wide range of "out-of-the-box" features. Setup options range from a simple template based store to the integration of a complex visual design and support for program modification.
AgoraCart offers a great deal of flexibility in many areas including full design controls through Cascading Style Sheets (css), template systems, customizable layouts, custom individual product category layouts and templates, customization of nearly all cart features for the code hobbyist, modular "drop in and go" code as well as AgoraScript, a scripting language inside parsed HTML pages, that experienced programmers can appreciate.
More information available on AgoraCart official web site.