Ideas

A shared rating of which food jars and tubs are easy to clean out and reuse

People who wash out jam jars and food tubs to reuse them find that some brands' labels peel off cleanly while others tear and leave sticky glue behind, and some products come in far more plastic than others. This would be a simple shared list where you look up a product and see how easy its container is to reuse, then add your own rating after trying it: did the label come off, is the lid worth keeping, how much extra packaging was there. Next time you shop, you could pick the brand with the friendlier jar. Products could be found by name or by scanning the barcode, using the free Open Food Facts product list.

Asked for by a commenter in a Hacker News thread

A crowdsourced aisle map of where items live in big warehouse stores

Warehouse stores rarely post aisle signs for products, so shoppers wander. Let people search an item and see the aisle and bay for their store, with community edits and recent-confirmation dates. Starts with a single chain and a map of the floor.