One shopping list, always current
Whatever you add at home is on the other phone at the shop.
Whatever either of you types is on both phones before the front door closes. Items sort themselves into aisles as you write them, so the list comes out in the order you walk the store.
Quantities stick, a typo is fixed in place instead of typed again, and crossing something off on one phone crosses it off on the other. Clearing the basket can be taken back, because thumbs slip.
The recipe book sends ingredients straight to the list, so cooking something on Thursday starts with two taps on Tuesday.
Free, on Google Play. Android only, for now.