I believe you are referring to an Oxy-Hydrogen generator as this is the only thing that you can actually add to a vehicle that will produce on the fly.
The way the oxy-hydrogen generator works, is it uses about 5-10 amps from your cars alternator to charge a mixture of Lye (KOH) and distilled water. This separates the hydrogen from the oxygen in the water and the hydrogen rises to the top and re-combines with oxygen creating oxy-hydrogen.
This is then fed into your air filter (after the filter) and into your combustion chamber whgere it mixes with the fuel and air and combust. Since hydrogen burns a little hotter this will give a little horsepower boost and you will get better gas mileage while reducing harmful emissions.
I have one on my Scion TC 2006 and it works just fine.
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If you are talking about a total hydrogen switch check out united nuclear, they make a system that can turn any late model vehicle into a hydrogen hybrid. They have a reall cool way of storing it: instead of having liquid hydrogen compressed, they use a dry storage chemical. The hydrogen combines witht he chemical and can be stored dry and not underpreasure until it is used.
Here is a blog about that product:
http://www.neutralexistence.com/blog/2007/03/car-hydrogen-conversion-kit/