Ever hear the name Stan Meyer? About eight thousand conspiracy theorists just nodded their heads. Stan invented a water electrolyzer - nothing new there - however, according to Stan anyway, his actually works on demand. The above video (c. 1990) shows him driving a VW-powered dune buggy running on nothing but water. Peruse YouTube for related videos, and you'll find he's been actually pretty well documented, but surprisingly unpublicized.
His method of electrolysis breaks the H2 from the O into their respective gases, both which are then sent to the modified fuel injectors and combusted in a regular old ICE. The only exhaust is more water vapor, which can be left to the atmosphere, or recycled back into the system. Of course, as I said, there have been many people to make electrolyzers that do that very thing, but not in the copious quantities that Stan's machine is said to.
When sifting through his notes and patent materials (.zip file available here), he apparently found the resonance frequency of water and tuned his electrolyzer with an oscillator to that frequency, vastly boosting the efficiency of the device, and making it an actually viable source of fuel, since you don't have to put so much more energy into it than you're getting out of it. No, this is not a free energy system, since energy from water is actually consumed, and water must be refilled, but the vastly more combustible gas produced nets much greater mpg of water than any means of burning gasoline.
Unfortunately, Mr. Meyer is no longer around to tell us about his invention - let alone sell us his proposed DIY kit for $1,500 - because he was poisoned. His brother, however, has continued his research to the best of his ability and posted Stan's notes and patent info. Stan was not a trained scientist or engineer, so his notes use terms not accepted by the scientific or engineering community, and they can be hard to decipher. I'm sure that if you can make sense of his research, prove theories correct, and build a working prototype, the world as a whole would really appreciate it. Post your comments, findings, thoughts and discussions for all to hear.
[Source: Water Powered Car, YouTube]