This is a photo series by Danish photographer Peter Funch. He staked out a few locations in downtown New York and shot hundreds of shots of people passing by. He then picked out people with similar characteristics, like wearing red, yawning, or carrying a yellow envelope, and photoshopped only those people into the image. For instance, everyone in the 'wearing red' photo actually passed by that part of New York, it was just a separate times. Funch merely went through, found them, and put them altogether to create the illusion that they were all in that place together at the same time. It's fairly simple, which makes it such a great idea.
Here's a couple more,
Everyone who was wearing black:
Everyone who was walking a dog:
I think it's amazing because the shots are all so beautifully composed since he probably had so many people to choose from. It's like setting up an image of twenty people without having to direct them. The series is called Babel Tales.
Here's his website: Peter Funch: Photographer