There is no viable way to keep track of how much I'm spending as I'm shopping. This is applicable to all types of stores, but in particular grocery stores where I'm usually buying at least a dozen items.
An app that helps users to keep track of their basket total, with the following features:
Add & edit an item with a user-assigned price
Automatic calculation of sales tax
Totalling everything up so you always know how much you're about to spend