This online database searches library catalogs all over the world simultaneously and then lists the results for each item by geographic area.

Another large, 30+ million union catalog that includes the holdings of many university, college, and public libraries. Several Philadelphia-area libraries are included in WorldCat as well as many seminaries and colleges and universities with religious affiliations.
Link: http://hdl.library.upenn.edu/1017/7158

Infotoday article about worldcat.org