Would "wind turbine generators" in the bypass stream save fuel for jetliners?
There are a few Alternators on big planes. 

Most large aircraft have an APU which is often in the back of the aircraft. It is effectively a small turbine directly bolted to an alternator. It provides power while the plane is on the ground (and the engines are off) and can also be used in flight if need be to supplement the main system or in the event of a main engine failure. 

The engine alternators are generally mounted and driven directly by the turbine (via a transmission) in this case near the compressor stage. You can read more in this thread and check out this video for a nice overview/explanation of jet accessory drives. You can also see where in this picture, 


(source)

As mentioned in the comments you can have a Ram Air Turbine driven by airflow but these are presumably not as efficient as directly mounting it to the engine.