Historical Map Viewer
From the USGS, naturally:
http://historicalmaps.arcgis.com/usgs/index.html.
Oh, what fun! Perusing old maps is part of my gig and this new map viewer is cartographic porn. Find your area of interest, your desired scale, click, and you've got a timeline of olde timey maps to look at. Check out how your neighborhood has changed over the decades.