Most large and medium sized U.S. and Canadian libraries are listed in the American Library Directory (Information Today, Inc.), available in print or online. Libweb links to the websites for thousands of libraries around the world.
U.S. Depository Libraries: The U.S. government provides government documents free-of-charge to select "depository" libraries so that these materials can be available to the public. You can locate Federal depository libraries using the Federal Depository Library Directory.
Many U.S. states also have depository libraries. You can generally find these libraries by calling the official State library (links are posted on Libweb).
Law Libraries: Most U.S. law libraries are listed in the AALL Directory & Handbook. See also the "Online Catalogs" section of this entry, below.
Special Libraries: The Encyclopedia of Associations (Gale) lists thousands of trade associations and non-profit organizations by topic and indicates which of them have libraries. The Encyclopedia is available as a multi-volume hard cover, on the Gale Directory Library and on Lexis (ENASSC).
State Libraries: Links to State libraries are posted on Libweb.
Online Catalogs: You can search thousands of library catalogs simultaneously through Worldcat. You can also find many library catalogs online by using your favorite search entinge, or through a meta-page, such as Libdex. Links to law library catalogs are posted WashLaw Web.