This is a few years old, but nifty as hell: Photographer Douglas Levere went around and re-shot many of Berenice Abbott's famous photos of New York from the 1930s. You can see some of the photos side by side here. Sometimes only a little's changed: a facade here, an elevated track there. Sometimes entire buildings have sprouted up. My favorite is the bakery on Bleecker Street that's still around today—even the way the bread's arranged in the window is the same as it was in 1937, but the buildings seen in the window reflection seem to have changed completely.

Ah, and there's a newer exhibit called Paris Changing, too. Different photographers, same concept. Now someone just has to do one for Washington, D.C.