While cleaning up a microscope recently, I needed to remove various threaded retaining rings. These are used in lens mounts, eyepieces etc. and normally have a couple of slots so they can be gripped with a screwdriver (or modified butter knife!). However, these rings didn't have any, so I cooked up a little tool to grip them internally.
A cylinder is turned on the lathe that fits closely inside the retaining ring, then the cylinder is split nearly to the end so the two halves can spring around slightly. A screw is threaded through one half and presses on the other, spreading them apart. By inserting the tool and tightening the screw, the inside of the ring can be gripped very securely, making it easy to remove. Here's a video:
Also see a similar tool for holding DC jacks.