Source: Claudio Saavedra on Flickr (edited). Licence.
ePub Books
Good for classic English and American literature.
Gallica
Very large collection of books and other material uploaded by the Bibliothèque nationale de France. Almost all material is in French. However you can search for material in a specific language, and the website itself can be viewed in several different languages.
Google Books
Good for older works that are out of copyright. Also offers brief extracts, cover images and contents pages for newer titles which are not offered full-text.
Internet Archive: texts
Large collection of mostly older works, on many subjects. Much non-scholarly material.
Library of Congress
Two collections, mostly but not exclusively with a US focus:
Marxists Internet Archive
Texts by Marxists or relevant to Marxism. No relation to the larger site called simply the Internet Archive (see above).
OAPEN (Open Access Publishing in European Networks)
Scholarly books, mainly in the humanities, including some important recent art books.
Open Library
Another large, very general collection of mostly older books, both scholarly and non-scholarly.