A supermarket in Oyster Card format. Every person has a card registered to them, and each item on the shelf has a micro chip instead of a bar code. Then all you do is put everything you want into your shopping bag and walk out the door through a scanner (such as the one for security tags now). The scanner totals up the cost of everything in your bag, deducts the cost from your card and away you go. No queuing in supermarkets ever again.