[kevinpet]

Specialty tools that fit vehicles you don't have. Specialty tools for operations that no one would perform by hand anymore. E.g. a cylinder honing tool.

Whitworth wrenches, sockets, taps, and dies. My dad had a British motorcycle shop around 1980 and I'm pretty sure there is still some of this in his tool boxes in the garage.

Homemade jigs whose purpose you can't even figure out.

I think the dominant category would just be extra or broken or cheap tools that aren't worth hanging on to. For some reason I have like four stubby Phillips #1 screwdrivers. Cheap hatchets with broken handles. Mushroomed lead mallets. A whole drawer of dull drill bits.

[drewzero1]

Ooh, those are good. I have a few homemade (or home-modified) specialty tools for my old cars, and some I'll certainly never have use for again.

Those are one reason I've never yet had a British or German car, though I do have one with a British-derived engine and a German fuel injection system. (I'll have to remember to pass on all the custom tools when I get rid of the car. The chances are slim to none that I'll ever own another one.)