Was recently shown this nifty site: Hyper Cities
They take google maps and then overlay historical maps on top of it. You can keep layering multiple maps and change transparencies and such. This has gotta be a useful teaching tool. It creates a mash-up of genius proportions. I love, love, love it.
If you checkout the Chicago map, you'll see that Soldier Field stadium was built on water! I mean imagine what this means if Chicago were to ever get flooded? Don't go to the stadium! But I suppose people already learned that lesson.