American Library Association: Professional library organization devoted to the protection and promotion of library services.
Illinois Library Association: Professional library organization devoted to the protection and promotion of library services.
Association of College and Research Libraries: A division of the American Library Association, ACRL is "dedicated to enhancing the ability of academic library and information professionals to serve the information needs of the higher education community and to improve learning, teaching, and research."
Library History Round Table: A division of the American Library Association dedicated to the history of libraries and librarianship.
Beta Phi Mu: International honor society for library and information studies.
British Library: The website often has online exhibits from its rich collections.
British Library Newspaper Library: The national archive collections in the United Kingdom of British and overseas newspapers. Located in Colindale (NW London), the library is reached via tube using the Edgware branch of the Northern Line. It's just a short block from the tube stop.
Library of Congress: Great online exhibits dealing with America's history and development.
Harry Ransom Humanities Research Center: One of the premier libraries in the U.S., the HRC has outstanding archival collections of some of the world's leading writers.
Bibliothèque nationale de France: Includes some really interesting virtual exhibits.