Collar stays (for searchability, they're sometimes called "picks", "shirt picks", "collar picks", etc.) for dress shirts. The little things that slide into the small pockets in dress shirt collars to keep them flat. Sure, you can buy these things in zillion packs out of better materials for almost nothing, but there are times when you might not have the right size on hand. And besides, you have a 3D printer. When you have a hammer, everything looks like a nail.