My hand-held light meter
Most of the photographers I know don't even own one of these beasts, probably wouldn't know how to use one if given one, or just rely on their built in camera meters.
That's fine, especially with modern SLRs both film and digital; in fact DSLR users may choose to rely mainly on their histogram info.
But that's not very practical for me, none of my film cameras have built in light meters as I favour totally manual operation.
For people using manual meterless cameras (like my Leica M4-P) only have 2 choices, either guess (using the sunny 16 method) or buy an external or hand held light meter.
My choice was the Minolta Spotmeter F.
The Spotmeter has a 1% angle of sensitivity that makes it perfect for the Zone System which is a very accurate way of controlling tones recorded on B&W film.