I collect manuals. I mean, not like actively finding them, but I never throw away the manuals that come with things that I have bought. So I have a stack of 20-30 manuals. Only started two years back, so looking forward to how they stack will grow in a decade or so.
I’ve sold some off over the years but I have some of my oldest models in my collection still.
Radio equipment... (tx and rx)... filters and other tools.
Books about programming...
Keyboards...