Consortia are associations of independent libraries that work together under formal agreements for resource sharing. This can allow for collective purchasing, document delivery, cataloguing services, and many other similar activities.
Union catalogues list the holdings of all the libraries in a library system.
OCLC - a nonprofit organization that provides computer-based cataloging, reference, resource sharing and preservation services. More than 72,000 libraries in 171 countries and territories around the world use OCLC services to locate, acquire, catalog, lend and preserve library materials
WorldCat (UBC Library subscription) and WorldCat.org (open) - union catalogue of over 10,000 libraries from around the world