A few more ideas:

I've used aluminium blind slats, cut up into plant labels (along with the ubiquitous yoghurt etc. pots for the same purpose).

I converted the chilled-water container from an old fridge into an interim reservoir in my greenhouse, for supplying water to capillary matting feed pipes/gutters.

Cocktail stirrers (like old biros, but better because slimmer) make good sticks for prodding holes for cuttings, etc.

Old nylon stockings/tights make very good tree/shrub ties - you can spread the presssure very widely, and they have a good combination of strength and stretchability; and they last for years outside. They can look unsightly, though - black ones seem best.

Perforated plastic bags some places sell bread in - these can be good propagators for cuttings (over pots) which requitre some 'closeness' of atmosphere, but with a little air flow.

I welded/brazed the two halves of a broken pair of old shears together, with some bends, to make a big draw-knife. This is for shaving the bark and shaping poles for making pergolas.

Matchboxes make good receptacles for interim storage of seed pods, while you wait for the seeds to be shed. Works only with small pods, of course (like violets and pansies). They allow a great enough passage of air so that the drying and seed-shedding is not stopped.

An old pair of jeans, with the legs cut off entirely (just leaving the waist and pockets) can make a handy tool belt.

Broad rubber bands, if sufficiently long, can make good grafting 'tape' (cut so as to break the loop, of course). I've used these on roses and maples with success. You can always paint the graft with rubber cement as well, if you want to improve the air seal (more likely on rough-barked plants). The rubber won't last much more than 12 months outside, though.