If I was to run an alternative fuel, CNG would be my choice right now for I have it piped to the house and I would want the option to run gasoline also (duel fuel), The home use slow feed compressors will be getting lower in price as the government pushes folks to go to alternative fuels.
I am looking in that direction even for my truck as I can tank up at home every night and can run on gasoline when needed.
There are a lot of city fleet auction cars and trucks around here that have me looking hard at them. Right now CNG is about 87 cents compared to a gallon of gasoline from what I have been reading, but the cost of the compressor and conversion to the car is right now a bit high unless you find the car or truck already converted, then its just a matter of the added cost of the compressor about 5 grand per car for a home slow feed (over night) system.
If I had a diesel, used french fry oil (bio-diesel) would have me looking hard at it as an alternative fuel, they say any diesel can run on it and it smells like french fries when running, NO JOKE. And from what I read, most fast food restaurants want to get rid of the stuff and pay to have it hauled away. For me the jury is still out on this one.