The Museum of the City of New York is digitizing its entire collection of photographs and is in the process of putting them all online. Base categories are by photographer, era, and borough. All of them are tagged and you can search for almost anything. I just did searches for Automats and the Bowery and Little Italy and Park Avenue and nightclubs. There are still some search result issues -- for example, I can't seem to see photos of Harlem ONLY from the 1930s -- but they're still working on the archive, so a filter of some sort might be implemented later. In the meantime, internet forever!