I can't believe I've not heard of this before. The David Rumsey Historical Map Collection is an incredible collection of maps from many time periods and geographic locations.

I especially like the 222 maps of the State of Maine.

The site makes it easy to search and find interesting maps. And then zoom in and out of them.

Thanks to All Things Maine for pointing out the recent addition of new Maine maps to the collection.

http://www.davidrumsey.com/

http://allthingsmaine.blogspot.com