It's called the Moment-O-Meter. It's not that big, and this device looks nice enough. It's a small plastic rectangular box that attaches to your windshield, and tells you to coast (green light) or hit the gas (red light) in order to maximize fuel efficiency thorough "sensible" driving.
Its website suggests savings of up to 50% in fuel used. The device is currently sold out over at Amazon.Com, but you can see a photo of the gadget there.
What's different about the Moment-O-Meter from the majority of fuel-saving gadgets that are so often ridiculed as being hoaxes by hucksters, is that the Moment-O-Meter isn't impacting the car, or the fuel, or the road -- it's impacting the DRIVER. (It was developed and patented by a retired aeronautical engineer, who created the machine to help his schoolteacher daughter with her gas budget.)
Bottom line, the Moment-O-Meter is encouraging behavior modification, because coasting, driving at lower speeds, as well as refraining from stop-and-start driving, really does save fuel. Sounds smart.