The most searched British META-OPAC. It contains the online catalogue of 22 of the largest university research libraries in the UK and Ireland, plus the British Library catalogue.
Karlsruhe Virtueller Katalog KVK
The wonderful University of Karlsruhe Virtual Catalogue allows you to search all the major libraries and national opacs in the world at once. Indispensable.
Maldura brings together online catalogues from Italian, Spanish, Swiss and Swedish libraries, plus the online catalogue of the British Library and the Library of Congress. Homepage is in Italian only, but the catalogue search engine is also available in English.