As Steve Matthews noted, modern cars will not use fuel while coasting downhill in gear. This is called Deceleration Fuel Cut-Off and uses various sensors to determine the engine load (i.e. the car is driving the engine) and whether the throttle is at idle (i.e. the driver's foot is off the pedal). The engine computer will then stop injecting fuel. Generally petrol engines continue to spark, but this consumes minimal energy.
This does not mean the car continues at a steady speed; it will commonly slow down because of mechanical losses in the drive train, and also because work is still being done compressing air on the compression stroke - without combustion, the air acts as a sort of spring, recovering much of this energy, but some heat is lost in the process. Vacuum in the intake manifold (around the throttle plate) of a petrol also causes strong engine braking effect. Diesels do not have throttle plates and their engine braking effect comes mostly from their much higher compression ratios.
Coasting along in gear will allow gravity to drive the car's ancillaries (oil pump, alternator). It also provides drastically greater control than neutral due to engine braking. You can also save fuel by using high-drain equipment such as heaters or air conditioning while descending the hill, as gravity will power these components. This does, of course, detract from the energy available to climb the next hill.
It should also be noted that this applies exclusively to fuel-injected cars; carburetted cars will continue to consume fuel. Additionally, 2-stroke vehicles cannot coast in gear without fuel, as this would starve the crankcase of lubricating oil. However, DFCO has been a feature since the early days of fuel injection - I own a fuel-injected car from the 1980s with this feature.