WhatDidI remembers what you bought recently, tracks favorites and finds deals. You can create a shopping list, which syncs to your Tesco account. Since you may not always have an Internet connection (i.e., iPod Touch at the market), the app can run in offline mode. Any changes you make are uploaded when you connect again.
It's been nearly a year since Tesco became the first grocery chain with an API. Though we've been unable to test WhatDidI, it looks like just the sort of app that we'll be using down the line. As we said last year: "Even if you don’t have a Tesco near you, pay attention. Someday your grocery store’s API could look just like theirs." The same could be true of this app.