Smartphone Charging Cables
You can never have enough charging cables for your devices. While your smartphone comes with one in the box, you usually need more than just one.
It's always nice to have one in the car and at the office as well, which means that anyone could use an extra cable as a gift. It makes a great stocking stuffer as well. You can buy Lightning cables for just $6 and microUSB cables come at many different price points, but Monoprice has some incredibly cheap ones available.