It is more wasteful to leave your car running, and it is also bad for the environment. Leaving your car running in park allows your car to continue to emit harmful matter into the environment, and will also waste gas. While it does take a bit of extra fuel to start your car, it is definitely a better idea to turn it off and on rather than to idle it in park.

The American Society of Mechanical Engineers found that it takes about six seconds' worth of gas (engine running idle, air conditioning on) to start up an engine. Therefore, if you are going to be idling for more than six seconds, it would save more gas to turn off the engine, and then re-start when you're ready to go.

This makes some sense when sitting at a drive-thru window, waiting to pick up a passenger, or pulling to the shoulder to use a cell phone. But it isn't safe to turn off your engine, say, at a red light - even though you're going to be sitting still for more than six seconds. You still need to be able to quickly avoid hazards whenever you're on the road.

Yes, it is almost always more wasteful to idle. Ars Cal mentioned the 6 second figure and although I have heard 10 seconds, the difference is negligible. If you are stopping, the odds are good that it will be for over 10 seconds. As long as it is safe, be sure to turn the car off when you are waiting.