According to the developers of a relatively new technology, you could be running your car on water by a simple modification of your car's engine. This modification will, in effect, convert your car into a hybrid, a vehicle pulling half of its power from water and the remaining half from regular gas.

How does it work?   A quart-size (95O cc) container is placed in the car, somewhere under the hood. You fill it with DISTILLED WATER and little bit of BAKING SODA. The device is supplied with vacuum and electricity (12 Volts) from the engine, and in turn produces HHO gas (Hydrogen+Oxygen). The HHO gas is supplied to the engine via any of the vacuum lines, which is connected from the engine's intake manifold or carb to the device. 

The device supplements your gasoline with a bit of hydrogen gas and extra oxygen. The hydrogen then helps your gasoline burn more efficiently, while producing its own combustion. That added combustion of the hydrogen gives you more power, and ultimatly requires less gasoline to run your engine, resulting, in better gas mileage.

It is claimed that the installation is so easy that anyone can do it.  The system is based on really basic electrochemistry and no fancy containers or tanks are need.  All you will need is some water, a small glass container and sone tubing.