The two massive magnets I picked up years ago, back when I was a fry cook. They were attached to the inside of some garbage can lids and they were used to separate flatware from the trash.
They got tossed when the (plastic) lids broke. I grabbed them.
They've saved dropped parts from cars, guns, bikes, etc. They've also picked up scrap, nails, steel cases, and other tire eaters from my driveways. I've used them to recover lots of stuff from under water, including three salvageable firearms and several wall-hangers.
Some very handy hardware for no real investment.