Grocery Gadget is a $2.99 list app that allows you to create custom shopping lists on your iPhone and then check off the items as you load your cart.
The app works as you would expect -- create a list by first selecting from a wide array of products (including, for example, rabbit food), and then check off the items one by one as you shop. Lists can be re-used by clicking a reset button - a good feature for repeat weekly trips to the supermarket. You can also add pictures next to items, a nice feature if you have someone shopping for you and you want to make sure they get the right thing.
The only real downside of this program is the price, which at $2.99 seems rather steep for something so basic as a shopping list utility. Also, it would be nice to add notes (other than a picture) to an item; right now, for example there is only an entry for "Beer" and no place to enter the specific type of beer.