One that everyone knows, I'm sure, is Matchbox Car boxes - the ones that hold 48 Matchbox cars - which you can buy for $5+, or used to, anyway - to hold cones of thread. I bought some at WalMart, then found an off-brand in a thrift store with cars in them for $5 a box. Gave the cars to the grandsons, kept the boxes for my thread.
I have a couple of dental tools I got from the dentist - too worn for his use, just right for cleaning out little places on old sewing machines. They were mine for asking. He saves them for hobbyists.
Disposable suture scissors, great for catching a single thread to cut.
Telescoping magnet from the hardware store for picking up things I drop
- as long as they're ferrous metal. Also picked up a mirror on a stick, like the kind the dentist uses, but it was in the hardware store.
Butler floss threaders for my serger.
Cross-locking tweezers - never know when they'll come in handy. Same with those wooden skewers. Or a medical clamp some people may know as a "roach holder."
Candy containers - I have bobbins in M&M tubes, hand-sewing needles in Tic-Tac boxes.
Tin containers -- Sucrets, Altoids, etc. I save those advertising magnets or buy sticky-back magnets, line the lid of the tin (be sure to leave room around the edge so the tin will close. I find them very handy for a thread clip or small scissors, thimble, needles, pins, a small tape measure, some thread, for a portable sewing kit. When I sew in another room, everything is in the box and I don't have to jump up and down five times. The magnet in the lid is insurance against getting a needle or pin in the chair or on the floor.
I'm sure there are more - I just can't think of them right now.