Library of Congress Subject Headings (LCSH) has been maintained since 1898 to catalog materials held at the national library. With cooperative cataloging, other libraries also use LCSH to provide subject access to their collections. 

Library of Congress Classification Outline, the letters and titles of the main classes of the national library's arrangement of materials.

WorldCat is the world’s most comprehensive database of library materials. Updated at a rate of nearly one new record every second, WorldCat contains bibliographic records and holdings contributed by more than 11,000 libraries around the world.

Includes some magazines.