Wind power

Look, here's a technology that's available to power vehicles solely by wind. Best of all, the technology is market-tested and ready for deployment today:

http://freakonomics.blogs.nytimes.com/2008/07/30/reports-of-sail-freights-demise-have-been-mildly-exaggerated/

Here's a more likely widespread use of the technology in practice:

http://news.bbc.co.uk/1/hi/world/europe/7205217.stm

I'm not quite there yet with my own vehicles, though with four babies, there have been times where the back seat of the minivan could have been referred to as the poop deck.