hydrogen and nuclear are the best answers, but thier obvious issues of supply and what to do with the left overs make them less that feasible.

What I don't hear people talking about is the power of corn. <img src="/images/graemlins/smile.gif" alt="" />

Distill the corn into ethenol. It has only about 2/3s the potential energy of gas, but it is renuable. The first Fords were ethanol powered, and I've seen cars modded to use it that are fairly recent.

For heavies, bio diesel. It is vegitable oil (say, corn oil) mixed with some ethenol or methanol, and lye. Lye can be leached from a lot of things, including the ash from burned corn stems and cobs.

Doesn't do much for air travel, but that is pretty waistful. Just a thought....