If you're looking for the perfect grocery list app, check out Grocery iQ (free, iPhone/iPod touch and iPad versions available, app2.me/227). It has every feature you might need, it's easy to use, it's available for both the iPhone and iPad, and it's free! Use this app once and you will never do your shopping without it. The best way to describe the app is to walk through how my wife and I use it.
Step 1: Enter the items you purchase regularly
After installing the app, enter your "Favorites" (i.e., the items you purchase regularly).