Your most useful tool that's not a tool
On and off for over 30 years I've made use of a long piece of steel pipe that started life as the outer part of a 3.0 Capri steering column.
If ever I need a bit more leverage on a ratchet or, more usually, to loosen wheel nuts overtightened by Kwiktwat, this magic bit of Ford tubing does the trick.
What non-tool item have you become reliant on over the years?