Alternative fuels, such as natural gas, continue to grow in popularity and use. Biofuels have already experienced substantial use on the market, and are likely to remain the dominant alternative fuel.
At its simplest, biofuel is "any fuel directly derived from recently existing biological organisms ... or agricultural byproducts as opposed to from petroleum." Since biofuels are grown and not mined, it is considered renewable in nature. Ethanol is a well-known biofuel.
Read more about the history and future of biofuels at the Energy Collective.