io9's playing with great opening sentences from science fiction. (More at MeFi.) Contrary to the aesthetics of io9's list, it seems to me that the best are those which refuse to immediately announce themselves as science fiction. Here are just a few from favorite s.f. novels that I haven't seen anywhere else (all links go to Amazon):
"We slept in what had once been the gymnasium."Though what can match the quiet elegance of Ursula K. Le Guin's The Dispossessed? "There was a wall."
—Margaret Atwood, The Handmaid's Tale
"Mars was empty before we came."
—Kim Stanley Robinson, Red Mars
"On those cloudy days, Robert Neville was never sure when sunset came, and sometimes they were in the streets before he could get back."
—Richard Matheson, I Am Legend
"What's it going to be then, eh?"
—Anthony Burgess, A Clockwork Orange
Unless of course it's Octavia Butler in Dawn: "Alive!"
QUICK UPDATE: I realized too late that I'd omitted a book that should be on any list of this sort, Olaf Staledon's Star Maker:
One night when I had tasted bitterness I went out on to the hill.