Depending on exactly where it comes from, the oil extracted from the oil palm (Elaeis guineensis) may be the worst fuel on Earth, insofar as it affects the climate. Once this oil is pressed from the fruit of the trees, it can be conversed into a form of biodiesel for use in internal combustion engines.
Nominally, biofuels are carbon neutral, as long as the same amount of biomass is being grown per unit time as fuel is being burned. The big problem with palm oil is that the plantations where it is produced (overwhelmingly in Indonesia and Malaysia) take the place of rainforests and peatlands that previously held massive amounts of carbon dioxide. As such, there is one gigantic burp of greenhouse gas when an area of forest becomes a palm oil plantation. This has been happening on an enormous scale, with the area under cultivation in Indonesia expanding from under 2,000 square kilometres in 1967 to over 30,000 square kilometres in 2000.
In addition to the climatic consequences, palm oil is a prime example of the food versus fuel debate. When food products are converted into vehicle fuels, they raise the price of those crops and increase the cost of food for those who depend on them. That effect is especially acute for the very poor, who spend a large proportion of their income on food. Palm oil is also found in 50% of all packaged supermarket products.
Quite probably, one appropriate approach would be for developed countries concerned about climate change to ban palm oil from former rainforest as an acceptable fuel. It is even worse than the very poor option of ethanol from corn, even before you take into account issues of international equity.