The most useful 3D prints I've downloaded (rather than designed) have got to be pegboard hooks and holders. There are a ton of different shapes and sizes, and they typically stay in the pegboard better than the metal hooks + plastic clip that you can buy at the hardware store.

Personally I don't print too many things made by other people. The whole point of a 3D printer is infinitly custom parts (within constraints).

So my process is usually running into a problem around the house and printing something to fix it. Or something I wished worked slightly differently and printing something to change it.

For example, I had drawer inserts that kept sliding back when I opened and closed the drawer, I printed a spacer to take up that room. My partner wants to feed his aquarium nori but I hate getting my fingers wet, so I printed a hanger for nori. My tea cabinet was disorganized so I designed a tesselated holder specifically design for the tins I use.

None of this is usually useful to other people, none of these came from other people. Let your printer set you free