A somewhat new technology based on the fact that diesel engines are very much like Mr. Fusion of Back To the Future Part II in that they can, with proper tuning, run on essentially anything containing hydrocarbons. This allows renewable resources, such as grain alcohol, vegetable oil, and methane from rotting garbage to be used as fuel. Many communities, notably the Bay Area of California are powering their public transportation using ordinary cooking oil. The eventual plan is to recycle used oil from restaurants for this purpose, but so far, the logistics of the situation have prevented this.
In an interesting side note, the exhaust of these vehicles smells exactly like french fries.