This photo mashup series by Shawn Clover that merges images from the 1906 earthquake and San Francisco today is a fantastic concept, with stunning execution.
From the artist:
To put these photos together, I first create a catalog of historical photos that look like they have potential to be blended. Unfortunately most of these photos end up on the digital cutting room floor because there’s simply no way to get the same photo today because either a building or a tree is in the way. Once I get a good location, I get everything lined up just right. My goal is to stand in the exact spot where the original photographer stood. Doing this needs to take into account equivalent focal length, how the lens was shifted, light conditions, etc. I take plenty of shots, each nudged around a bit at each location. Just moving one foot to the left changes everything.
I am in awe.