H2O Water Powered Car (Reuters)
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A Japanese company has invented an electric-powered and environmentally friendly car with an electric engine that runs on water. The basic power generation mechanism of the new system is said to be similar to that of a normal fuel cell. The key to the system is its membrane electrode assembly, which contains a material that's capable of breaking down water into hydrogen and oxygen through a ... (continued below)

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...chemical reaction. Genepax, the Japanese company unveiled the 300W eco-friendly car with a demonstration on 13 June 2008 in Osaka, Japan. The cost of the energy generator (not including the car), once it gets into mass production, is said to be about $5,000 (US). The technology is explained in this diagram. The big question is, what is the cost of the the metal hydride that is consumed in the process?
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UPDATE (from Wikipedia): In June 2008, Japanese company Genepax unveiled a car which it claims runs on only water and air,[25] and many news outlets dubbed the vehicle a "water-fuel car".[26] The company says it "cannot [reveal] the core part of this invention,” yet,[27] but it has disclosed that the system uses an onboard energy generator (a "membrane electrode assembly") to extract the hydrogen using a "mechanism which is similar to the method in which hydrogen is produced by a reaction of metal hydride and water".[28] The hydrogen is then used to generate energy to run the car. This has led to speculation that the metal hydride is consumed in the process and is the ultimate source of the car's energy, making the car a hydride-fuelled "hydrogen on demand" vehicle, rather than water-fuelled as claimed.[29][30][31] On the company's website the energy source is explained only with the words "Chemical reaction".[32] The science and technology magazine Popular Mechanics has described Genepax's claims as "Rubbish."[33] The vehicle that Genepax demonstrated to the press in 2008 was a REVAi electric car, manufactured in India and sold in the UK as the G-Wiz. In early 2009, Genepax announced they were closing their website, citing large development costs.[34]