This topic is all over the popular science magazines. The website www.livescience.com has new info almost daily about this.
People have known for a long time that sewage produces methane. Methane is a worse greenhouse gas than CO2, but when it is burned it is a very clean fuel. If cities used bacteria to ferment sewage, they could generate quite a bit of methane. I'm clueless as to why this isn't done on a large scale.
Any biological material decomposes, and most animal products and vegetation give off a variety of gases that can be used as fuel. Just think of all the leaves that are raked off people's lawns every autumn - this is an enormous mass that could easily be treated with water and enzymes to make methane.
There are some credits and grants. Ethanol production is being subsidized by the government, but there are serious problems with producing and using ethanol. Ethanol is not as toxic as gasoline but it's not as good a fuel either, and so people have to use more of it. Turning over millions of acres to grow corn to ferment into ethanol might help us make a dent in importing foreign oil, but we could probably achieve a greater reduction by better engines.