One of my favorite pieces of design is the London Underground Map. It has been replicated all over the world – from Mumbai to Tokyo. Leigh Wolf just sent me a link to a BBC 4 video made in 1987 about this map.
Check it out here
Here is a link to the Wikipedia page about the map, and two versions of the evolution of the map. The first is a webpage giving links to various versions of the map and the other is a Youtube Video.
Here is a fantastic page giving a varied and rich history and context and variations of the London Tube map. Well worth a visit.