Biofuels -- fuels made from biological raw materials -- continue to be the focus of intense research, but so far they haven't come close to replacing petroleum and petroleum diesel made from crude oil. Because different fuels have different energy content per gallon, you can't simply compare the price-per-gallon, but instead need to compare the average price-per-unit-of-energy. And in late 2010 in the United States, gasoline cost on average about $2.78 per unit, while ethanol cost about $3.45 for the same amount. However, you'd pay about $2.86 for biodiesel [source: U.S. Department of Energy]. While biofuels may one day become inexpensive fuel sources that are better for the environment than fossil fuels, they also present challenges: The production of some low-cost biofuels may cause unintended environmental damage. Researchers are exploring "next-generation" feedstocks such as algae and grasses, but these are still expensive to produce. In the meantime, there are biofuels available that are relatively easy on the wallet and we'll explore five of them here.