The David Rumsey Historical Map Collection, started in the 1980s and digitized gradually over the last 10 years, now has over 18,000 maps available online.
One of the maps, Montreal 1815, can be overlaid with varying transparency on Google Maps to demonstrate the tiny boundaries of 1815 Montreal compared to today’s metropolis.
There are so many ways to view this massive collection I can’t even begin to make a recommendation (the site itself could use some maps) so start at the home page and get lost.