On a recent trip to NYC, we passed through Grand Central Station a couple times. It's a very cool space - very austere yet commercial (Apple Store) and of course there are people rushing from place to place actually using it as a train station to boot.
The slow shutter speed (I think the image above was about a second and a half) renders those with places to go as ghosts quickly passing through.
This longer exposure (six seconds) below thinned the crowd even further as those on the move became just dark spots moving across the floor with only tourists and observers remaining. The longer exposure of course did manage to catch a couple of folks taking pictures of me taking pictures of them and so on.