Alternative energy sources—from algae to cow manure—that are really out there.

The United Nations' Food and Agriculture Organization has looked at cassava, a potatolike crop grown across the developing world, as a possible feedstock for biofuel. Also known as tapioca and yucca, cassava is drought-resistant and needs less fertilizer than other crops, making it cheaper than corn.

Estimated production cost: $1.40 to $2.40/gallon.

Prospects: Moderate. Growing cassava for fuel could drive up food prices, either directly or by diverting land away from other crops. But developing countries may be eager to support a homegrown energy source.

Because it grows quickly, has a high oil content, and needs only sunlight and water, algae looks promising as a source of both ethanol and biodiesel. It also serves as a filter for dirty water and as a carbon sink. Ideally, an algae farm could be located downstream from a large-scale farm or factory, where it can clean the water of pesticides, carbon, and heavy metals.

Estimated production cost: $1 to $2/gallon.

Prospects: Good. Algae is cheap and easy to grow.

Thanks to the mountain pine beetle, some 500 million cubic meters of British Columbia's lodgepole pine forest have been turned into a hole-riddled tinderbox. The province's Lignol Energy Corp. is developing technology to turn the beetle-infested timber into ethanol. The job's made easier by the insects' own handiwork, which leaves the trees easier to break down.

Estimated cost: $1.50/gallon

Prospects: Moderate. Using trees for fuel will always risk pushback from environmentalists.

In 2004, the Central Vermont Public Service launched the Cow Power program, which pays dairy farmers to produce fuel in the form of methane, made from cow manure through a process called anaerobic digestion. There are 135 anaerobic digesters operating in the United States, according to the EPA. Those digesters produce enough energy to power some 25,000 homes.

Estimated production cost: Varied.

Prospects: Excellent. Anaerobic digesters are already widespread in Europe.

Oklahoma-based Syntroleum Corp. converts chicken fat into synthetic fuel, using a process it calls hydro-processing. The company says the fuel produced from chicken fat is chemically identical to regular, petroleum-based fuels.

Estimated production cost: Less than $2.40/gallon.

Prospects: Good. Barring an explosion in vegetarianism, otherwise-useless chicken fat will continue to be scraped off the floor of America's industrial-size rendering plants for the foreseeable future.