Google Books - A repository of millions of digital books and magazines. A majority of titles published before 1923 are available in full-text, with many of those still in copyright indexed and searchable.
The Internet Archive - A free repository of the full-text of public domain books, currently numbering more than 23 million, as well as films, music and audio recordings.
University of Pittsburgh Press Digital Editions - The full-text of more than 750 out-of-print titles from the University of Pittsburgh Press on a wide variety of academic disciplines.