Take that app that lets you compare product prices by taking pictures of the bar code (smartshop or something like that?) and let it simply track your purchases while you're in a store. You tell it what store you're in, and as you throw things into your shopping cart, you first take a picture of the bar code and then tell it how many you're buying. It should be able to track sales tax, etc.
It should also have a mode where you can use items in your pantry, or items from online databases, to build a shopping list. Bonus if you can tell it, over time, where in the store that item is located, so that when you come back for future shopping trips, it will order your shopping list accordingly. That way you don't have to hunt back and forth from aisle to aisle to make sure you got everything on your list (nor will you have a bunch of missed items at the end of your trip, forcing you to go back through the store to find them), nor do you have to scan through your list to make sure you got everything from the current aisle.