Panic, the Mac software company that makes Coda and Transmit, has an apparel store with fun shopping cart behaviors that mimic the Mac's dock. The apparel store shows t-shirt listings, and a little cart dock at the bottom of the page that invites visitors to drag t-shirts down into it to add to cart. Clicking and dragging shows the ghosted t-shirt moving with the cursor until it snaps into the dock. Adding a bunch of t-shirts enables the carousel. When you drag t-shirts out of the dock, they disappear with a puff of smoke, just as items in the Mac dock do. Those who don't like to drag can use the plus sign icon.

The cart page shows a clean and simple form with all of your items to purchase, your information (shipping/billing info), and payment info. Short and without any fuss. I wish open source carts were easy enough to configure to be this simple.