There is one where you put magnets on the fuel line. It is supposed to "line up the molecules" and therefore improve performance and economy. I'm sure it is down to psychology. You put it on the car and believe it will save fuel, so subconsciously you drive in a way that does.
Jackie Stewart's racing driving school used a ball in a saucer. If you drive smoothly, accelerate smoothly and brake smoothly, the ball stays in the saucer. Keeping the ball in the saucer in corners scrubs off less speed. Jackie Stewart got James May to adopt the style and he actually drove faster. But the principle goes for saving fuel as well, the system has to use more fuel to accelerate suddenly compared to gradual accelaration, even if you get to the same speed in the same time.