This is 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.
PORBASE - Base Nacional de Dados Bibliográficos
PORBASE is the largest bibliographical database in Portugal: it lists all the collections of the Biblioteca Nacional as well as of 149 other libraries in the country (University libraries 63%; specialized libraries 19%; other public libraries 10% and private libraries 8%). All these libraries can be accessed through the Directório de Bibliotecas Cooperantes da PORBASE run by the BN.
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. This is an indispensable resource.