I have several, mostly because I can't afford non-cheap tools.
One are those cheap plastic snap-knives from Home Depot and elsewhere.
They are sharp, they weigh nothing, they lock reasonably securely, and the blades are quick to deploy. I use them as marking knives for furniture building. The line superior to a pencil line, thinner but indelible. The only issue is that the blades rust in humid environments.
Another is largish masonry nails.
They use tougher, harder steel than ordinary nails. A masonry nail about three inches long makes a great cheap nail set. They can also be ground into driver bits. To take apart a vacuum cleaner once, I needed a triangular bit (not a tri-tip). I made one quickly from a masonry nail with my dremel tool, and it worked perfectly.
Another is bamboo chopsticks.
They are strong, resilient, and splinter-free. They make great stirrers and pokers, and can be used as dowels or wooden nails. But they have to be bamboo. The even cheaper white wood ones are splintery and fragile.
Another is a largish hunk of thickish plate glass. (no image) I got mine from a broken coffee table somebody was throwing out. I ran an old belt sander belt over the edges to blunt them. It is about 12" x 5". It isn't as flat as an engineer's granite block, but it is pretty close. I use it as a flattening plate for my waterstones. I hear people use them with wet-or-dry sandpaper as sharpening stones. Simple water makes the paper stick. (I heard of one guy using a piece of slate from a discarded pool table that way too.)
Baling wire, or any relatively thin steel wire, is pretty much essential.
It can be used for fastening, bundling, clamping, repairing, etc. It is much stronger than string or cord, and doesn't stretch. Use stainless steel wire for outdoors. Note: simply twisting the two ends together will not make a secure connection. Look on YouTube for videos on how to use it.
Lastly, old hard drives have impressively powerful magnets inside. You will need a set of small Torx bits to take them apart. The magnets are usually attached to a thickish steel plate that can be removed, with difficulty. I usually leave the plate on. Sometimes I cut off some of it with a dremel tool.