Library of Congress Digital Collections
loc.gov/collections
Browse and search the Library of Congress's digitized collections, spanning maps, photographs, manuscripts, films, sound recordings, and printed works.
Why it is useful
The Library of Congress holds one of the largest collections of primary source material in the world, and an increasing portion is freely available at high resolution. Unlike the World Digital Library which focuses on international materials, the core LOC collections are depth-first on American history, culture, and government. The maps and photograph collections in particular are extraordinary.