Readers may be interested in the Smithsonian Institution's Collections Search Center website, which allows visitors to "search over 5.4 million records with 460,000 images, video and sound files, electronic journals, and other resources from the Smithsonian's museums, archives & libraries." One can search by typing in a term or by using the headings provided, including "Type," "Place," "Name," "Culture," or "Datasource." Each of the headings is further subdivided; "Type," for instance, includes books, paintings, trade catalogs, and a huge number of other types of material. Just as an example, a simple search on "railroads," limited to results with online media, calls up nearly 2,000 items; one on "steel" returns over 4,400.
Tip of the hat to the American Historical Association blog.