minimal shopping cart/ ecommerces sytem that's gpl'ed?

i've been looking around for a shopping cart system that's super minimal (ie doesn't get in the way of the rest of the site) and provides a user friendly interface for changing prices/adding product and categories. does anyone have any suggestions?

I could write something, but i'm not all the quick with php, and the page i'm currently working on doesn't have the budget for that. any suggestions are welcome.