Library of Congress

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:

http://www.loc.gov/about/

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.