With everything from algae to used french fry oil, engineers have been able to get cars going on fuels other than oil and gas. I even read an article recently on a car powered by wind.
But even though these alternative sources are fun to explore and may work, they aren't necessarily economically feasible. I recently heard on the radio that BP is planning on running cars on algae for the next Olympics. Cool, but paying $75 per gallon? Not so much. Which leads me to ask you what you consider to be the fuel of choice for cars of the future.
What alternative energy source do you think has the best balance between being environmentally friendly and cost competitive?