A plumb bob is a handy tool, but until recently hasn't been improved much since the time of the Egyptians. A modern update that's a lot more convenient to use is this Plumb-Rite by Tajima.

The protruding knob you see is actually the butt end of a setting pin. To hang the tool, you strike the knob with a hammer to drive the pin into wood or sheetrock.

The line stands exactly 50mm away from the surface of whatever the tool's mounted to. (They also make an imperial variant where the line is exactly two inches away.)

The mounting surface is also magnetic.

While you cannot see it well in the photos, there's also a little diamond-shaped recess on the mounting face, beneath the exit hole for the pin. This recess allows you to hang the tool from a protruding fastener.

The line itself can auto-retract, and you can also set the tension on it, so that you can pull the bob to whatever length you'd like and it moves no further. This also works when you're adjusting the bob upwards. Lastly, the yellow plastic thingy on the butt end of the bob supposedly helps dampen the swaying.

I've cued this video to the part covering this particular tool. As you'll see, a lot of thought went into the UX:

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