Found this on Lifehacker:
Use WorldCat to Search Libraries
The next time you need to find something at your local library, try searching through WorldCat, a ginormous network of libraries, library content, and library services. You can use WorldCat to search for everything you would search for at your local library, find free content (downloadable audiobooks, for example), articles and citations, historical documents, and many more kinds of information from libraries all over the world. You can also use WorldCat to check out items directly from libraries you have an active membership with (this can vary by library).