screwdrivers
I do computer repairs and servicing, and my business partner and I visit customers' premises a lot. Screwdrivers are consumables to us, like toner, ink, pens, whatever. No matter how many we buy, I never have one in my van when I need one, or the workshop. They just seem to go missing. So every time we order a Draytek router for a customer, we order a half dozen of their promotional screwdrivers. They come with two flat blade sizes and two pozis and they're only a quid each.
Perhaps they're related to biroid life forms....