I've used old galvanised iron cold-water pipes as the uprights for espaliered fruit trees. No rust, rot or (hopefully) leaching. Also easy to drill wires through.
They make great stakes too.
I find some of my handiest tools are basically bits and bobs lying around: clothes pegs, butter knives, chopsticks, short lengths of bamboo...

I use long lengths of pipe to hold down garden cloth. You can roll the cloth or plastic around the pipe, then pin it in place with "J" shaped hooks pushed into the soil. It's much faster than installing multiple pins or burying the edges.

I have a drum from an old commercial clothes dryer that I made into a roller. After I prep my beds for seeding, I go over them with the roller, which makes a flat, level surface. It's big and awkward, but really makes the beds look good, and the seeder works beautifully.

I use aircraft cable and turnbuckles all over the place for creating solid supports for trellises, including concrete reinforcing wire.

I use slats from window blinds for plant and row markers. They are easy to cut to size, last for years, cost nothing but a little labor to salvage and fabricate, and are easy to re-use.

I am a great lover of tools. Once you've collected a few hoes, you begin to understand why so many different designs have evolved. I like the Japanese Ho-mi with a short handle, its shape and design make it very versatile, but I've found that for really detailed, close-in hoeing, like in a densely planted English-style kitchen garden, a large clam shell, what you see commonly used for ashtrays around here, is just about perfect.

Those political signs in people's yards. The ones I will be offering to dispose of after the polls close on Tuesday. They make great supports for a plastic sheet over the garden if needed. Also good tie offs for stray tomato vines. The metal ones they use around here can be reshaped with a couple of vise grips and some elbow grease.

1. Bench grinder/Dremel tool/Metal file - I've used all three and still use all three to keep my tools sharp, sharp, sharp. I use my scuffle hoe on a daily basis and as soon as I feel some resistance while weeding, off to the sharpening bench. Digging is a breeze with a razor sharp shovel.

2. Rope ratchets - use these to quickly (and easily) raise and lower growing lights. I have an assortment of failed solutions cluttering up my workshop, so these are a welcome addition.

3. Old rubber car mats - when working close to the ground in wet soil, these keep your knees dry and somewhat clean. I still haven't found a way to eliminate working on my knees!

4. 2L Pop bottles (that would be 70oz Soda bottles for our neighbours down south) - the clear plastic variety. I cut off the bottom three inches or so and use them to cloche the early brassicas.

Probably sounds silly, but I've found this mini-shovel very useful. Sometimes the handle of a standard shovel or spade just gets in the way.

Here is a link that might be useful: Mini Shovel

m lorne - good call on the sharpening tools, I use the same trio.

You should try a piece of ground-contact insulation for a kneeling platform. The foam will compact eventually, but it gives a good dry soft elevated and warm space to kneel or sit while you're working, and easy to come by at construction sites.

timcad - I use those too! Just don't remove them till after the election... people are awfully touchy about that, and lots of them are armed these days.

Boards of different widths, that match seed spacing recommendations. Put the board on the bed, tip it at 45 degrees, slide it back and forth - instant straight furrow, and V-shaped so the seeds are all at the same depth and in a clear line. Flip it over, do it again, second furrow, exactly parallel and exactly the spacing width you want. Repeat till you run out of bed, seeds, or knees.

I like my garden mattock and my pointed, cultivating hoe (haven't tried those other hoes)

And a 5 gallon bucket :-)