Woo can be found in all sorts of different places. One that has been around for a while now is the motor engine add-on purported to get more fuel economy from the engine. Sadly, they do not always work (BBC). Or perhaps ever at all.

C'mon guys, if it was that easy to get 30% better fuel economy from a petrol engine, the engine manufacturers would already be doing it.

(The one examined works on the principle of using electricity from the car battery to split water into oxygen and hydrogen, and feeding that in through the air intake. Which may indeed produce a smidgeon more power, but requires more power to stop the battery going flat. Any patent for the process would be thrown out on the basis of it being a lightly disguised perpetual motion machine.)