In 1666, most of London burned to the ground. The old city as it once stood, from the buildings to the streetplan itself, are lost to history. Oh, unless you take a look at this video.
Built using old maps and drawings of the city which predate the fire, a team of students from De Montfort University in Leicester (in conjunction with the British Library, GameCity and Crytek) have rebuilt the British capital in the CryEngine, giving us the kind of look at the old London historians have previously only been able to dream of.
It's not playable, but if somebody wants to get the map off them and turn it into some kind of Thief mod, that would be awesome.