City Resource hosts maps of all kinds, mostly those which intend to make an argument. For instance, the 1966 Map of Mumbai (Bombay), when overlaid on the current OSM, one can see how the mud flats are being built upon, with no flood mitigation plan in place. Below are some useful web maps showing Bombay's evolution.
(click on the map to view the webmap in a new window)
To see all historical maps as webmaps on a single page, click here
To Draw on a 1926 map of Bombay click here