It's called water4gas . They tell you a lot about it on their site and they give seven free lessons. They have two books for sale that aren't very expensive, that take you through the conversion step by step, but they also say that if you have mechanical savvy, you can probably do the conversion without the books. It seems that it's only an addition to your original engine and entirely reversible. The parts needed will cost approximately $60.
Anyway, if you're interested just click on the link above and read all about it for yourself and make up your own mind. If it works as well as they say, it will cut down on the amount of fuel you use and clean up your car's emissions, as well as making it run better.
I know how many of you are struggling at the moment and I hope that this does work. I showed it to my mechanic and he says that it seems feasible, but I really want to stress that I know nothing about these sort of things and encourage you to make your own decision.