Damn near everything that is related to the US in some way, from historical government documents to public domain things like photos, videos, music and other media from the well documented to the totally obscure.
“The Library serves as the research arm of Congress and is recognized as the national library of the United States. Its collections comprise the world’s most comprehensive record of human creativity and knowledge. Open to those age 16 and older without charge or special permission, it is the world’s largest library and a great resource to scholars and researchers.”
The school my dad graduated from makes it a habit to have the doctoral students make two bound copies of their thesis. One for the university library and one for the Library of Congress. I don’t know if this is a common practice.