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