I was reading an article about new batteries that run off of sugar water. Similar principle to a fuel cell, except using a fuel that neither leaks through its container or explodes. Apparently can run on almost any sugary liquid (not Coke, though -- the carbonation screws it up).
Then, to top that, Sachin sent me a link to a car that runs on air. How much cheaper than that can you get? Of course, with a top speed of 40mph, it's not going to get you anywhere in a hurry.