I’m reading Dragon Keeper by Robin Hobb right now, and I have to say I really like the world Hobb has created with the ships of magic and the Rain Wilds and the dragons.  I was talking to a friend about writers who have created unforgettable worlds.  

For me there’s Middle Earth and Darkover.  Hogwarts is a favorite.  George has, of course, created a massive world.  I notice that most of these apart from Darkover are fantasies, and you could read Darkover as fantasy — mediaval trappings, psionic powers that seem like magic, etc.  

Are there equivalent worlds in science fiction?  I admit I’m drawing a blank right now which is odd since I generally prefer S.F. over fantasy.  I can think of individual favorite S.F. novels, but not a whole world.  Maybe that sense of place as a character is unique to fantasy?