Wowio: Contains a limited variety of full online texts and comic books. Also includes a purchase option. Registration is required.
Booklamp: Will examine the text of a book and recommend similar books based on various criteria within limits set by the user. Does for books what the Pandora online music service does for music. This is currently in a beta status and is not complete. Registration is required.
Google Books: Contains a wide variety of incomplete online texts. Also includes a purchase option. Useful for examining a book before purchasing. Registration is not required.
Multiple
Bartleby.com: Includes reference sources from encyclopedias, dictionaries, thesauri, and quotations
