I'm surprised nobody has mentioned algae-produced biodiesel yet - it's
vastly more energy productive than land based crops!
Soy-beans can produce about 55 gallons of oil per acre per year and oil-seed rape about 100 gallons per year.
But that's nothing compared to algae - take the same acre in an algae growing pond and they can churn out up to 10,000 gallons of oil per year!
This was the amount a UNH group achieved while looking into the feasibility of making algae derived biodiesel (
see here for details). They were planning to use the agricultural waste streams that flow into the Salton sea as the source of fertilizer.
However, since the government canned the funding of that project (dare I say it threatened the supremacy of the oil companies?), small startups have realised the potential of algae and are setting up enclosed ponds in sunny locations and are reporting very favourable progress. Some project that their algae derived biodiesel will be cheaper than crude in the near future.