Cover Browser: Created in 2006, Cover Browser displays galleries of comic book (and book, pulp, games, DVD or magazine) covers for comic book fans like myself to explore & enjoy (there's also links to buy comics). At the moment, there are 455,613 searchable covers from 2,923 different series available.
50 Books/50 Covers 2008 selections: AIGA
Czech Book Covers of the 1920's and 1930's: Smithsonian
Book Cover Creator: Create your own book cover