Anything that's been fit to print at:

 telnet locis.loc.gov
Not a sexy interface, but it's pretty comprehensive :-)
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 As of September 2007, the Copyright Office Catalog is no longer
 available through the telnet protocol.  To search the catalog, please
 use your web browser to connect to www.copyright.gov/records.
 Thank you.
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Any one can come inside and read, and you can register as a reader which lets you order items from the stacks to read on-site (you can't take them out unless you're a member of Congress). Living near Washington, I've found it an invaluable resource for research. -- DavidBrantley

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Also, the short name of the classification system the LibraryOfCongress uses. (See LibraryOfCongressSystem.) It's more complex than the DeweyDecimalSystem, and is popular with university libraries.

Also a popular measure of electronic capacities - "...stores the equivalent of '''ten Libraries of Congress''' on a chip thinner than a '''human hair'''".