I read a few articles about a hydrogen boost system for gas engines. Basically, a mini electolysis machine to create hydrogen and pump it into the intake. Anyone played with this? They claim 50-60 mpg. If you want to read it, search for hydrogen boost, several sites will show up.
I knew a guy that built such, and is now in the process of marketing it. It was called the hydro-power pack. He claimed it would get 25% better economy, and burn cleaner, gas or diesel. There are a few around that have used them that said the same thing, I haven't had my hands on one yet, the company he was supposed to be marketing it through wasn't too helpful.