Oxford Dictionary of Twentieth Century Quotations

Oxford Dictionary of Quotations
-from Oxford Reference Online

Oxford Dictionary of Modern Quotations
-from Oxford Reference Online

Bartleby.com : Great Books Online
Includes standard reference works, and works of prose, fiction, and non-fiction.

Oxford English Dictionary
Complete text of the 20-volume Second Edition, first published in 1989, with its 3-volume Additions Series, published in 1993 (vols 1 and 2) and 1997 (vol 3). Draft material for the upcoming 3rd edition (2010) also included.