Finding locations - online resources

The British Library - all online BL catalogues

Union catalogues:

Access to Archives (A2A)

COPAC (national academic)

Encore (sets of performance music in the UK)

FAB libraries

LibraryWales

Irish public libraries ("Borrow Books")

Search25 (London)

The National Archives integrated open-source catalogue

NWL ( North West Libraries Interlending Partnership)

Northern Ireland Libraries

OpenDOAR (Directory of Open Access Repositories)

UK Union catalog of Chinese books

UnityUK

Access to individual catalogues:

UK Public Libraries on the web
University of Birmingham
University of Manchester
University of Sheffield

Public library consortia:

Libraries West
London Libraries Consortium
South East Library Management System

NHS / Health library catalogues:

Base-Library
Nursing Union lists of journals
Shelcat (Scottish Health Libraries Catalogue)
SHINE (Scottish Health Information Network)
South West Health Libraries Network
West Midlands Healthcare Libraries Network 

Other library catalogues:

British Film Institute
British Library for Developmental Studies
The Cathedral Libraries Catalogue
Westminster Music Library
V & A - National Art Library

International interlending
Information and resources for international interlending, including WorldCat

FIL(The Forum for Interlending and Information Delivery)
Very useful page listing a wide variety of online catalogues