Yup. They've been around since at least the early 70's, when everyone had an uncle who invented a 150mpg carburator that was suppressed by the oil companies

I live less than 10 miles from the water fuel museum (seriously). Apparently you can burn oxygen and hydrogen to produce more energy than it took to separate them

. In fact, the claim was that a small battery could be used to start the process of splitting the water molecules (that much is actually true), and recombining them would liberate enough energy to not only drive the car, but also to drive an alternator to continue the process

Since the combustion product is water, I guess you could just let it run forever on just the initial battery charge and water supply, creating more and more energy from, uh, from, well .... This miracle was suppressed by the oil companies too

Now, where did I put my tinfoil hat?
UPDATE: The museum closed in August, 2006, but you can still buy a book, DVD of interviews, and plans