Not sure if this one falls in the "holy-water-in-the-radiator" category but this thing called Hot InaZma is popular now this summer in Japan among car enthusiasts.
There is no way to know what's in the box but it is a device that is attached to the anode and cathode of the battery. It claims to increase torque in addition to the effect provided by earth cables.
they sell 3 type:
-LR (low revs , city driving)
-HR (high revs, sport driving)
-MR (multi-purpose)
Car Goods Parts magazine tested it and found a fuel economy improvement of about 15% on a BMW.
Is there something similar out there that was proved to work?
Hot InaZma