Association of Research Libraries
http://www.arl.org
Choose "Member Libraries" and click a letter or scroll down to find university library web sites across North America, arranged alphabetically.

The Library of Congress Web Site
http://www.loc.gov
The Library of Congress collection includes more than 120 million items. You can search its catalog here.

LibWeb
http://sunsite.berkeley.edu/Libweb/
Offers information on library holdings in over 100 countries.

The New York Public Library
http://www.nypl.org/index.html
Click "Online Resources" to begin searching the catalog.

Smithsonian Institution Libraries
http://www.sil.si.edu/
From this site you can search the Smithsonian's 20 libraries and 9 archives or research centers. Click "Digital Library" or "Libraries" to get started.

Western European Library Catalogs on the Internet
http://www.lib.virginia.edu/wess/eurocats.html
Portal site to individual libraries and to directories of libraries and archives in 17 countries.