Here is a collection of vintage Penguin Books covers that will make you wet your pants. This set was put together in response to the totally must-have Penguin by Design: A Cover Story by Phil Baines (softcover, hardcover).
I found this via BoingBoing, who also recently linked a long interview with Penguin art director Paul Buckley (part 1, part 2). Buckley headed up a project to revamp the original covers with designs from people like Chris Ware (who did Candide), Art Spiegelman, Seth, and Charles Burns. Tons of pictures in the interview. Awesomeness all around.