You've seen the pulp covers featuring wide-eyed women clad in sparkles and not much else. But The Haunted Lamp, a blog which features "The Eerie, The Sublime, And The Little Bit Queer," has collected some of scifi's beef-cakiest covers.
Take note that the Astounding Science Fiction cover is apparently "Inappropriate." But still Astounding. More astonishment at the link. Oh, and the cover with the naked man on it? It could be NSFW, depending on where you work. [The Haunted Lamp]