One option which i have been putting a lot of time into is Joomla, you can get Joomla with VirtueMart built in. You can download the combined package from the VirtueMart website here
VirtueMart: Your free e-commerce solution. - Welcome...
As soon as i installed it and start playing with it i realized that the shopping cart part itself makes this package really good, and i am in the process of setting it up for my own website. Unfortunately i don't have one with the cart setup properly online at the moment for you to have a look at but my Orion Networks site in the footer is running off the install of this package, and i am planning to get the cart running within the next week or two.