About three years ago I had a chance to drive a Prius. Once I was in the car and driving it, I realized that the single biggest thing the Prius did to save fuel was to have the engine turned off when you do not need it.
I decided to adapt one aspect of this to my driving - I turn off my car at traffic lights. When I am driving in the city I am amazed at how long I sit at lights. There are some rather long ones in Victoria and the benefit is clear - I can save as much as 300 ml at some of the lights.
This issue was the subject of an editorial on CFAX today with the station coming out against the idea because it would make no difference.
I have been doing this most of the time for about three years now. I have not seen any issues with my starter, that was my initial concern. I estimate that I am saving about 200 litres a year doing this at the moment, more than enough to buy a new starter every six months at current prices.
I turn on my car an extra six times a day on average because of this. Without this I would be turning the car on average six times a day total.
I have researched the issue as much as is possible - google hypermiling for extreme tips on how to save gas. I have not found any data that points to what I am doing as being bad for the car or not being useful in saving fuel. The only issue I can not answer with certainty is the time one needs to have an engine off to save an fuel. I have seen all manner of estimates, the best I can find is that anything more than five seconds will save you fuel.
One argument against it is that I am not in control of my car - well, I am in control, it is not moving. No stationary car has ever caused or been the reason for an accident. All accidents are caused by moving cars. Slowing your car down and coming to a stop is always the best option in a dangerous situation - yes, I am sure someone can come up with some odd situation where this would not be the case, but they are reaching and I am sure I could solve the same situation by stopping the car.