This video by Will Gallia shows what a day on London's Tube looks like. It's a dizzying glimpse into the daily life of a mass transit system, from the frantic pace of rush hour to the orderly quiet of a train at midnight.
Gallia made it by compiling 562,145 trips over the course of a week in 2009 (a 5 percent sample of all rides that week). Because Tube riders use their Oyster cards to swipe in and out of the subway, it's possible to track (and visualize) trips on the system. In Gallia's visualization, each journey lasts about .003 seconds in order to fit them all in.
Transport for London (TFL) has used data in interesting ways before. It releases some data for study, and results have included a map of bicycle traffic. TFL also provides a list of available data, including the Oyster card journey information.
(Thanks to Gizmodo for the link.)