Anything that's been fit to print at:
telnet locis.loc.gov
Not a sexy interface, but it's pretty comprehensive :-)
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.
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
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".