- WorldCat - a catalog of books and other materials in OCLC member libraries worldwide.
- Library of Congress Catalog - a catalog of books and other materials in the Library of Congress.
- National and State Libraries - a list of national libraries worldwide
- Copac - a union catalog providing access to the merged online catalogs of over 70 UK and Irish academic, national & specialist libraries, including the British Library, the National Library of Scotland, and the National Library of Wales
- The European Library - a union catalog providing access to the collections of the 48 National Libraries of Europe and leading European research libraries
- Greater Western Library Alliance - The Greater Western Library Alliance (GWLA) is a consortium of 31 research libraries located in the Midwest & Western U. S.