The near future for Natural Gas (methane), which at present is a clean-burning domestic fuel,  promises a solidly sustainable fuel. 

Why?
Well, in metropolitan areas, organic matter such
as food & fecal waste is  concentrated, due to population density.  Both of these waste
streams, when decomposed without oxygen (anaerobic digestion), produce methane. This idea of
waste equals food/fuel is a premise of sustainability, astutely
outlined in William McDonough's classic, 'Cradle to Cradle'.

Currently, natural gas is quite popular in the transit and refuse sectors, due to a variety of factors, including lower cost of fuel, reduced need for engine maintenance, and drastically reduced emissions.  In the light-duty sector, the Honda Civic GX (the only OEM NatGas vehicle for purchase in the US) has been named the greenest car in America for five years running!  If the GX does not meet your needs, know that  Baytech Corporation and BAF Technologies are certified by the US EPA to perform warranted retrofits on General Motors and Ford vehicles.  Whether, OEM or retrofitted, Natural Gas Vehicles (and fuel) are currently eligible for large tax credit.  The only current issue is lack of refueling options, although for personal use, that can be remedied with the purchase of a Phil home refueling appliance, which connects into your existing natural gas line.
However, with the coming realization that we in Ohio, home of 3 major metro areas, can produce our own fuel from our own waste; plentiful refueling options are destined to be on the way!

Ohio's examples of Natural Gas from Waste:
Akron Sewage Plant >
Solid Waste Authority of Central Ohio >

Natural Gas Vehicle update via Green Car Congress:

The US Environmental Protection Agency (EPA) has certified NaturalDrive Partners, LLC’s dedicated compressed natural gas (CNG) retrofit of 2008 Chevrolet Impala sedans powered by 3.5-liter and 3.9-liter V6 engines in models LS, LT, LTZ, as well as law enforcement packages 9C1 and 9C3.

The retrofitted CNG Impala reduces NOx and CO emissions more than 50% compared to gasoline the gasoline versions, and offers about a 15% reduction in CO2 compared to gasoline. With the 3.5L engine, fuel economy is 19 mpgge city and 30 mpgge highway. The retrofit on the LTZ model also leverages GM’s Active Fuel Management (cylinder deactivation) for superior fuel economy.