Tom Carden has put together a couple of interactive maps of the London Underground system that measures distance by the time of travel. Clicking on a station will alter either map to reflect the time it takes to travel to any other station. The first map moves the stations around; the second retains the geometry of the underground network and builds time contours around it. The first is probably more dramatic but the second is probably more usable. Both are Java applets.

By way of Great Map.