A CodeIgniter-Powered & Open Source E-Commerce Package: GoCart
GoCart is an open source e-commerce application which is focused on offering an easy to use/customize solution and not having a huge number of features but only the important ones.
It can have unlimited categories, tiered categories with any depth, unlimited products-product images-options and can display related items.
The built-in shipping methods include a flat or table rate, USPS, FedEx + UPS and has support for Paypal, Authorize.net and "COD (collect on delivery)" payment methods.
Users can have multiple addresses with the help of an address book and use giftcards or coupons with the system.
A plain admin panel empowers the store owner with features like the ability to configure categories, products, export user + sales data, get sales reports, implement promotions and create/edit content within the site with a simple CMS.