Cheap barcode scanner + Atmel AVR + strong refrigerator magnet. The first time you scan your product, it memorizes the date. Every time after, it indicates the

Cheap barcode scanner + Atmel AVR + strong refrigerator magnet. The first time you scan your product, it memorizes the date. Every time after, it indicates the product age with leds, sound, or an LCD. You keep it stuck to your fridge, scan groceries going in/being opened, scan anything questionable again before use to check the age. You'd have to keep a sheet of one-off barcodes for stuff like restaurant leftovers, fruit, etc. As the userbase grows, you could support downloadable tables of expiration dates for various foods(barcodes) so you wouldn't have to even think about that. Somebody could probably make "a bundle" by mass-marketing a gadget like this, but that would make me angry. Thinking like a user rather than an entrepreneur here, I'd prefer to see it as a cheap, hackable, perhaps Arduino compatible electronics kit on Sparkfun.com or the like.