If you wanna cart that you can integrate into an existing site, then take a look at Avactis shopping cart. (You can download the free limited demo and play with it) It's made right that way. It consists of modules. So when I needed some part of the storefront to display on another page I just added the corresponding PHP tag into the right place, and it worked. I could also change the look of separate categories. It's also possible to change a single product's template, but I just didn't need it. So I guess it may fit your needs
Here's another helpful link: http://www.shopping-cart-reviews.com/