You shouldn't sit at a light with your car in neutral. I couldn't tell you if it saves anything worth while in gas, or not. What I can tell you is that it just isn't SAFE.
If you get rear ended your car has the ability to roll forward (or backward if hit head on). With the clutch engaged, the car will come to a hault.
Through my experiences driving from NY to FL and back, MANY times - cruise control has always increased my gas mileage.