For a real alternative fuel option, woodgas conversions are about as alternative as you can get. This Ford F150 has a woodgas generator in the bed and is a current example of some pretty old technology. It was built by a guy just to see if he could make it work, which it does.
Woodgas had a period of popularity around WWII when gasoline rationing meant you either parked the car or found some alternative fuel for your engine. Some European countries at the time, Denmark is one great example, used woodgas in almost 95% of their vehicles; farm machinery, tractors, trucks, fishing boats, even stationary engines, you name it. The widespread use shows it definitely works but it's also inconvenient and not really an option unless all other fuels are unavailable.
Woodgas is one type of producer gas, the result of controlled and incomplete combustion of some carbon material such as wood, many types of biomass or charcoal among others. The gas given off consists of hydrogen, carbon monoxide and methane, among the combustible components, the rest being primarily nitrogen. When this woodgas is fed into the engine you can drive down the road like you would with any other fuel, except for the rather cumbersome gas generator you need either attached to your vehicle or on a trailer along with its fuel supply, plus the lower power produced makes driving a more utilitarian pursuit. It works, but it's slow and finicky.
The photo below shows an example of an Adler Diplomat conversion back in WWII, which also indicates the size of the gas generator on the back of the vehicle, plus bags of wood fuel on the roof. Interesting technology and necessary at the time but again, not something you would want for your regular commute.
If you know what to look for, you'll see woodgas powered vehicles in movies about WWII, usually a pickup truck but sometimes a car will have one of these contraptions on the back. Neat technology and not something well known these days. I think it's pretty cool.
Adler Diplomat with wood gas generator
Link: Ford woodgas conversion on eBay - listing has ended