This collection is an old favorite of mine. The Society In Decline Project is a catalogue of those old signs you see on the road. Built in the early part of the century, they have long ago lived out the most useful part of their lives and are a testament to the decay of small parts of our country.
One of my old favorite collections is this set of black and white photographs taken in Manchester in the 1970’s. Manchester was in the midst of a decades long fall from its height as the manufacturing center of Britain, in a way that is very analogous to what is happening to Detroit today. The flickr set has comments, and if you have time it is worth going through and learning a little bit about the decay of a great city.