Many cars sit in line waiting to leave parking lots, use facilities, drive-up windows, etc. I am curious about how much gas per minute an ordinary can uses while idling.

Here is one opinion on the topic:
Re: How much gas does a car use when idling, how much used to start the engine?

If one were inclined to know the value for a specific vehicle, it might be possible to get a fuel consumption value in real time with software that communicates with the engine control unit via the OBDII port. For VW's and Audi's, the VAG-COM product may do this. I'm sure that similar applications exist for other makes.

This may be way more involved than you intended... :D