Update:
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Google skipped right past the third dimension and landed directly in the fourth (time) by offering historical maps on Google Earth. Now you can travel back in time -- for example, I am looking at the globe of 1790. Don't expect detailed high resolution photography from days gone by, but it's still interesting to see old maps overlaid on the satellite imagery of today.
Playing with layer transparency on the overlaid maps gives you a good sense of how things have changed over the years -- especially when looking at more detailed maps like New York 1836 or London 1843.
Currently, maps are available for:
Update:
To use this new feature, expand the Featured Content -> Rumsey Historical Maps in the Layers panel.