And now for something completely different comes a home-made spot welder by Albert van Dalen. His reason for making a spot welder is to weld tabs to rechargeable battery cells in order to construct larger packs - commercial welding machines are expensive thus Albert's welder was born.
It's quite a feat for home-made engineering, and can either be used in series or opposed configuration. An Arduino is used to act as a timer for the welding duration, and the electronics are quite minial compared to the hardware required. The only caveat is that some heavy mains voltage and current is required as well as working with higher voltages, so not a project for the absolute beginner. However if you're tempted, a quick demonstration is shown in the following video: