“One day, I shall come back. Yes, I shall come back. Until then, there must be no regrets, no tears, no anxieties. Just go forward in all your beliefs and prove to me that I am not mistaken in mine”
“Interfere? Of course we should interfere. Always do what you’re best at, that’s what I say.”
“Reverse the polarity of the neutron flow.”
“He was a hundred and seventy days dying and not yet dead.”
“Poets have tried to describe Ankh-Morpork. They have failed. Perhaps it’s the sheer zestful vitality of the place, or maybe it’s just that a city with a million inhabitants and no sewers is rather robust for poets, who prefer daffodils and no wonder." - Terry Pratchett, Mort
“Hammerfall!”
Or would something from The Mote in God’s Eye be more appropriate?
The latter, near the door. One of these days I’m going to build my earth-sheltered home (which will have a round door, even if I have to build it myself), and that’s going on a plaque just inside it.
For another quote that should lend itself well to artistic representation:
“We are star-stuff.” -Delenn, Babylon 5
"This is Ripley, last survivor of the Nostromo, signing off. "
Perhaps a bit of a stretch, but:
“Did you ever wonder what it would be like to go swimming out among the stars?”
“Men who live in a highly advanced society experience not life but the image of life, on ever and ever higher levels of abstraction and dislocation from reality. If left to their own devices, they invent strange professions such as the critic, the critic who criticizes criticism, and even the critic who criticizes criticism of criticism. It is a very sad misuuse of human talent and energy.” - Jack Vance, The Book of Dream
A really good Mass Effect quote:
“Does this unit have a soul?”
Babylon 5 is rich with quotes.
But probably my favourite quote is from John Brunner’s The Shockwave Rider, and it’s my sig line.
“I used to think it was awful that life was so unfair. Then I thought, ‘wouldn’t it be much worse if life were fair, and all the terrible things that happen to us come because we actually deserve them?’ So now I take great comfort in the general hostility and unfairness of the universe.”
“The next time - the next time you want a revelation - could you possibly find a way - that isn’t quite so - uncomfortable?”
“I’d like to live just long enough to be there when they cut off your head and stick it on a pike as a warning to the next ten generations that some favors come with too high a price. I would look up at your lifeless eyes and wave like this. Can you and your associates arrange this for me, Mr. Morden?”
Damn you, Morgyn! 
Sorry if you guys have already posted some of these
Think of it as evolution in action.
Any sufficiently advanced technology is indistinguishable from magic.
What good is a glass dagger?
East takes you out, out takes you west, west takes you in, and in takes you east.
Not all those who wander are lost.
Deserves it! I daresay he does. Many that live deserve death. And some that die deserve life. Can you give it to them? Then do not be too eager to deal out death in judgement. For even the very wise cannot see all ends.
Then the whole world will be barbarian until men learn a new way to coerce nature, and the swordsmen, the damned stupid swordsmen, will win after all.
I’ll stop now.
Risk is our business.
Logical, flawlessly logical.
My name is Mister Dark, of Cougar and Dark’s traveling Circus sideshow.
My name is Talking Tina, and I love you.
“What smells purple?” last line of The Man with English by H. L. Gold
Others last lines here.
“I know where I came from—but where did all you zombies come from?”
It was a bright cold day in April, and the clocks were striking thirteen.
Starting from where we are we go 33,600 turbo miles due up. Then west in an arc-deviation to here. Then following the Great Circle several radiolubes south by down-east. By astro-astralmov to here then here. The by space navico-cumpus to here and here and then to here and here. Then 13 point strato-cumulus going 4 million light years and THUS to our destination.
Violence is the last refuge of the incompetent.
“Anyone who clings to the historically untrue — and thoroughly immoral — doctrine that “violence never solves anything” I would advise to conjure up the ghosts of Napoleon Bonaparte and of the Duke of Wellington and let them debate it. The ghost of Hitler could referee, and the jury might well be the Dodo, the Great Auk, and the Passenger Pigeon.”
The enemy’s gate is down.
“He loved Big Brother” – 1984
From Dune:
The beginning is a very delicate time.
I see many things. I see plans within plans.
He who can destroy a thing, controls a thing.
Usul, we have wormsign the likes of which even God has never seen.
“And I thought they smelled bad… pant …on the outside!”
“All these worlds are yours except Europa.
Attempt no landings there.”
“Dead or alive, you’re coming with me.”
“One point twenty-one jiggawatts! It worked!”
“If it bleeds, we can kill it.”
“They come out at night. Mostly.”
“Nuke it from orbit. It’s the only way to be sure.”
“It is a strange fate that we should suffer so much fear and doubt over so small a thing. Such a little thing.”
“The needs of the many, outweigh the needs of the few.”
“This is important. It means something.”
“Look around! Can you fashion some sort of rudimentary lathe?”
“The whole principle [of censorship] is wrong. It’s like demanding that grown men live on skim milk because the baby can’t have steak.” — Robert Heinlein
“A human being should be able to change a diaper, plan an invasion, butcher a hog, con a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects.”—Heinlein
“Man fears the darkness, and so he scrapes away at the edges of it with fire.”—Rei Ayanami, Neon Genesis Evangelion
“Isn’t it worth the loss of a hand to have created the workers of the
future — the machine men?”
—Rotwang, Metropolis
“Civilized men are more discourteous than savages because they know they can be impolite without having their skulls split, as a general thing.”—Robert E. Howard
“The time for using the knife to remove this cancer is long gone. Bring forth the torch.”—Exarch Quaillindral, Warhammer 40000
Suspension of disbelief does not mean hanging it by the neck until dead.
-Attributed to Marion Zimmer Bradley
“If it can’t be expressed in figures, it is not science; it is opinion.” — Robert Heinlein
“Comic books easily distinguish nerds from jocks—nerds read epic conflicts of good vs. evil, while jocks memorize decimal statistics about whichever team plays closest to their house.”—Cracked