Old Book Illustrations is a search engine and browseable library of—you've guessed it!—the engraved illustrations and litho prints found in old books.
Choose the type of illustrations you want to see: animals or people, landscapes, buildings, etc. Choose your favorite illustrators from a list: Gustave Doré, John Leech, Charles H. Bennett…[and] Find illustrations by the title of the book or periodical in which they were published: Æsop's fables, Punch, L'Illustration…
The scans are high-resolution (though it appears the scanned items are sometimes worse-for-wear) and most come with lots of details about their original creation and printing.