Simply, the most excellent website ever I have found is over at MapJunction.com.
Once you open up the Java applet, you can select from a huge number of maps of Boston – current day, but also all the way back to 1775. You can overlay one map with another, to see how things used to look, where buildings used to be, where streets used to be, etc. It is absolutely awesome.
For example, if you want to know exactly how Boston was laid out, before they filled in the Back Bay and South End, select a map from the 1850s and one from today, and you can see the outline of the city, then and now.
Beyond the wow factor, I actually found a use for it, albeit with someone who was kind of paranoid. A client of a friend of mine was looking to buy a condo. However, he had heard that, once in a while, basement units in the South End could flood (which is true, although fortunately only about once every 100 years). So, he decided that the only way to be sure that the basement didn’t flood was to buy a condo that was on real land, not landfill. (In reality, the way to make sure the basement doesn’t have flooding is to put in a sump-pump, and/or don’t buy a basement unit.)
So, I simply printed out the maps, one overlaying the other, and, with a magic-marker, outlined where the original Boston shoreline was (it follows Washington Street). This meant anything within the lines was the old Boston, and everything else was landfill.
I’m sure I could have written this all a lot better, but you get the general idea.