CNN has video of a car designed to run mostly on water. According to the video, the engine uses a small amount of gasoline to power a process to split the water into hydrogen and oxygen. The engine then burns the hydrogen for power. And according to the designer, Anthony Brown of Jacksonville, his car gets 100 miles per ounce of water.
Don't get too excited, though. I'd like to see this independently verified and tested before I run out and convert my car. Plus, he says he gets 100 miles per ounce of water, but he doesn't say how much gasoline the car uses in that process.