Shopping Carts

OS Commerce
http://www.oscommerce.com/
A leading open source shopping cart platform coded in php. OS Commerce has a large developer community and is widely used.

CRE Loaded
http://www.creloaded.com/
A derivative of OS Commerce. Community support forums.

Zen Cart
http://www.zen-cart.com/
A derivative of OS Commerce.

X-Cart
http://www.x-cart.com/
PHP shopping cart. Offers an affordable custom development service rather than much of a community. Uses Smarty templates.

Click Cart Pro
http://www.clickcartpro.co.uk/
PHP shopping cart with ready-made skins.

Commerce Starter Kit for ASP.net 2.0
http://www.commercestarterkit.org/
An open source shopping cart for ASP.net 2.0.

www.aspdotnetstorefront.com
asp.net shopping cart.

CubeCart
http://www.cubecart.com/
Limited functionality in the free version, but pay for the full version and it's a very capable shopping cart. PHP and MySql based. Easy template system.

Mals eCommerce
http://www.mals-e.com/
Free, simple and easy, integrates with a number of payment processors.

Lite Commerce
http://www.litecommerce.com/
A lightweight high-tech ecommerce solution devoted to making lauching an internet store easy.

.netCART
http://www.dotnetcart.com/
A full featured ASP.NET shopping cart and ecommerce component.