http://www.hydrorunner.com/
Interesting product, it uses some of the electricity generated by the engine to split water into hydrogen and oxygen, which is then sucked into the intake. I wonder if something like this will end up being common on trains and trucks.
At the current gas prices it would still take years to pay for itself (the website claims it costs $3500, but doesn't say whether this includes installation), so you would probably have to keep it each time you sold your car to make it worthwhile.