Ooooh, cleaning products.
Mr. Clean Magic Eraser. Not for general cleaning so much as marks, stains, anything that "won't" come off. It took black marker off a wall, crayon off a linoleum floor, and 5 years of gray gunk off patio furniture, all of which had been previously scrubbed senseless with a variety of cleaning products.
As with so many other things, the ads made it look gimmicky, and it wasn't until a woman I know demonstrated it for me--
because she carries one in her purse at all times simply for the purpose of convincing other people how amazing it is--that I really understood.