I'll start this, let's see where it goes.
In mine:
A plastic mirror 60mm x 170mm about the same size as the rear-view mirror from an 1972 Austin Allegro, no frame, no backing, nothing - very useful for looking into places where you cannot otherwise see. I also have a dentist's magnifying mirror on the end of a stick - but that is less useful as it distorts.
One thing I did when I was at The Science Museum building stuff, was that any cabinets that were built to house AV/IT/ Other tech which had sockets at the back, the back wall of said cabinets would be lined with plastic mirror. It made connecting/disconnecting stuff a whole lot easier, when you aren't driving blind.
1m of clear silicone tubing with an I.D. of ~4mm. Very useful (utilising lung suction power) for retrieving dropped small components from places where the flexi-claw-grab cannot be operated. It tends to rely on the component having a reasonably flat surface for it to seal against - although the flexibility/capacity-to-conform-to-shape of the silicone helps. It'll happily retrieve an M8 washer.
Bits of old credit cards, bent into a 'U' shape, for inserting between brake shoes and rim when applying or adjusting the toe-in of the shoes.
Waiter's friend corkscrew. For those moments when things have gone really badly awry. But, I suppose, that's a proper tool. Like the one which caused it to be deployed.