The Library of Congress is the largest library in the world, but it's much more than a collection of materials used by the members of Congress:
It includes the U.S.
Copyright Office, which registers and collects copyrighted works, along
with a Law Library and the Congressional Research Service. In addition,
it provides cataloging information and services which assist libraries
all around the country. Much as you could spend days roaming the
Library's physical stacks, you can also spend days examining its online
collections. Happy exploring.