All those wonderful plastic coated wires from DS's toy packages (cut them down to size for twist ties, or leave as is for larger ties). Invariably, these have plastic pieces with predrilled holes in them. I'm sure those can be used for something eventually.

Plastic coated tags, sized right, are a great base for making your own duct tape flat packs, and easier to come by sometimes than old gift cards.

Old gift cards (see above).

Assorted mint, candy, and medicine tins (need I say more).

Almost any container with a lid gets cleaned up and saved for use somewhere else. Plastic peanut butter jars make great all purpose containers.

We recycle cans and cardboard so there's usually plenty hiding around somewhere when needed for other uses.

The list goes on with what others have mentioned: wire hangars, plastic grocery bags, paper bags with attached handles, packing materials (especially bubble wrap - I used a bunch to wrap christmas decorations for storage in the attic).

Trooper, nice call on the shredded paper to be reused for packing material.