Cab Tracker: Averages the last four hours of cab routes into a map, then lays over that the routes of ten cabs in real time. Yes, you can now sit at your desk and watch cabs travel over San Francisco, brought to you by GPS. No, you won't know who's in them, which is probably for the best, or where they're going, or how much of a tip the driver got. Ultimately, it's about the visualization of GPS data.
We'll never look at a Yellow Cab the same way, if only because we know someone else is looking at it's ghostly GPS trail. This skein of data is also available for artists and works they've created from it can be seen on the site.