Here is one that I did not know. I have drill bits that go from 1mm up to 10mm in 0.5mm increments. I needed to drill a 12mm hole in a panel that I had fabricated ready to take a grommet for a cable.
I checked in with a neighbour and he only had 10mm drill bit maximum as well, so I got online to order a 12mm and a 32mm one as well for another cable hole next to it.
Here is something called a hole cutter that comes as a pack of 3 allowing holes to be made in 2mm increments from 4mm up to 32mm. All 3 came in a little pouch for about £10.
And they worked so quickly I was rather surprised:
The 12mm hole on the right for the heater cable took about a second to cut from 10mm to 12mm. The one on the left needs the bigger cutter!
Inexpensive way of making lots of different sized holes!