propfan MD-80 

I read that during a period of unusually high fuel costs, MD built one -80 with propfan engines (these look like turbofans, but with a propellor attached to the rear; they are known for their effeicency). After some initial hype, no orders came, and the model (I believe it was a one-off, more of a demonstrator than anything else), was scrapped. There has been intermitent talk of using propfans on civil aircraft since then, but it has not come to anyhting.