WorldCat

WorldCat is the world's largest networkof library content and services.
WorldCat libraries are dedicated to providing access to their resources on the Web, where most people start their search for information.

The National Library of New Zealand will add some 8 million records and the holdings of 275 libraries to WorldCat, the world's richest database and resource for discovery of materials held in libraries around the globe. As a result, records of items held in New Zealand libraries will be visible to Web searchers worldwide through the OCLC WorldCat.org service, or through popular search engines such as Google and Yahoo!
See more information here.

You can put the WorldCat search box on your website. Just register and copy the HTML code.