Penguin has been publishing high-quality paperbacks of classic science fiction since the late 1950s. As times change, you can see how the book covers move from stately red and black (above) to crazed psychedelia (below).
I love that the title of Pierre Boulle's now-famous novel La Planète des Singes was originally translated as Monkey Planet. Doesn't quite have the same ring to it without the apes.
John Wyndham, ever fond of crazy monsters covered in tentacles, seemed to get the best covers even in the staid 1950s.
I absolutely love that Pohl cover with the spaceship made out of a finger. It's sort of a Terry Gilliam look - the space opera that gives you the finger.
You can see all these covers and many more, with detailed explanations of how they came about along with information on the art directors and artists involved, at The Art of Penguin Science Fiction website.