Shorpy's great. Another good one (and larger than Shorpy, I believe) is the livejournal community Found Photos.
The library of congress has some great stuff, too. Library of Congress Prints and Photographs Online Catalog.
If you're ever searching for some reference for urchins, search their archives for the Nat. Child labor committee collection. The majority of the photos are terribly sad, but still, they're great reference.