Oxford English Dictionary, JSTOR (online versions of articles), Grove Art Online, Index to Theses, Ingenta Connect (online journals), International Medieval Bibliography, Iter Bibliography, MLA International Bibliography, and much more.
COPAC: an invaluable online resource that merges the catalogues of most major libraries in the UK and Ireland. Through COPAC, you can, for instance, locate copies of books in London libraries other than UCL.
MultiOpac. An Italian version of COPAC for locating books in Italian libraries.