If you haven't already visited Google Earth, you're really missing out. Sure, you can actually visit places in person, but this is one of the best alternatives. That is, assuming you don't want visit random places in the middle of nowhere, which can't be seen anyway in the program.
So those of you who have seen Google Earth are probably asking, "Dude, Google Earth is so ... 2004." Think again, surfer dude. Now, you can view historical maps on Google Earth. I guess that means the program is so 1790 ... or whatever historical map you choose.
You also have the ability to use the layer transparency to display something like New York 1836 over top of present-day New York city. I expect these to look slightly different, especially without all of the taxis.