The Smithsonian certainly has some superb graphic collections, and I think one of the best and most powerful from a graphic point of view, is the collection of Seed Catalogs from 1830 to present.
In total there are about 10,000 catalogs in the collection, from which about 500 superb images are available online.
They provide such a fascinating window into the past in so many ways – including but not limited to Graphic Design, Packaging, Botany, Advertising, and Colour History.
The gems in the collection are those from the 1830′s to 1930′s – here is just a small sample. Well worth spending hours viewing them all.