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.
The most common Italian META-OPAC is SBN which is run by the National Library Service. It contains all the main libraries in Italy. It may also be useful to consult META-OPACs organized by region, such as MAI (META-OPAC AZALAI Italiano), which collects all Italian OPACS and allows you to search in three different ways:
META-OPAC AZALAI Catalogo Bibliografico Virtuale Università Lombarde
This includes catalogues of all Lombard Universities.
This is the collective catalogue of libraries in Lombardy.
This is the collective catalogue of Tuscan libraries.
Similarly, collective catalogues for specific materials or field of studies could be extremely useful, such as:
Metaopac for Periodicals ACNP
This is the national catalogue for journals and periodicals.
This is the census of Italian 16th-century editions.
This includes catalogues of libraries of architecture.
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. It is truly indispensable.