This is not a complete list. I don't always take quotes while I'm reading; these are ones I've grabbed when the opportunity has presented itself. As I'm always rereading Pratchett, this list will probably grow over time.
The Colour of Magic
"I'm going out of my mind . . .
( mind speaks ) Good idea, its getting pretty crowded in here..."
The Light Fantastic
"The pen is mightier than the sword if the sword is very short, and the pen is very sharp."
"Drinking that glass of water would mean genocide for thousands of innocent germs."
"Inside every sane person, there's a madman struggling to get out."
"He did of course sometimes have people horribly tortured to death, but this was considered to be perfectly acceptable behavior for a civic ruler and generally approved of by the overwhelming majority of citizens. [Footnote: The overwhelming majority of citizens being defined in this case as everyone not currently hanging upside down over a scorpion pit]"
"What do you call those things you find at the bottom of rivers?
Frogs? Stones? Unsuccessful Gangsters?"
"I meant," said Ipslore bitterly, "what is there in this world that truly makes living worth while?" Death thought about it "Cats," he said eventually, "Cats are Nice."
"Many things went on at Unseen University and, regrettably, teaching had to be one of them. The faculty had long ago confronted this fact and had perfected various devices for avoiding it. But this was perfectly all right because, to be fair, so had the students."
"What's so surprising about bacon? I dunno, I suppose it comes as something a shock to the pig."
"Whatever happens, they say afterwards, it must have been Fate. People are always a little confused about this, as they are in the case of miracles. When someone is saved from certain death by a strange concatenation of circumstances, they say that's a miracle. But of course if someone is killed by a freak chain of events: the oil just spilled there, the safety fence just broke there: that must also be a miracle. Just because it's not nice doesn't mean its not miraculous."
"You could end up being totally, oddly sad and full of compassion which would lead you to believe that it might be a good idea to wipe out the whole human race and start again with amoebas."
"Corporal Nobbs, why are you kicking people when they are down?"
"Safest way, sir."
"He was determined to discover the underlying logic behind the universe- which was going to be hard, because there wasn't one."
"You didn't have to go and kick me?"
"You're quite right, it was an entirely voluntary action on my part"
"The consensus seemed to be that if really large numbers of men were sent to storm the mountain, then enough might survive the rocks to take the citadel. This is essentially the basis of all military thinking"
"The Gods Of The Disc have never bothered much about judging the souls of the dead, and so people only go to hell if that's where they believe, in their deepest hearts, that they deserve to go. Which they can't do if they don't know about it. This explains why it is important to shoot missionaries on sight."
"Camels have a very democratic approach to the human race, they hate everyone of us, without making distinctions for rank or creed."
"The most prudent thing any intelligent animal can do, if it would prefer its descendents not to spend a lot of time on a slab with electrodes clamped to their brains or sticking mines on the bottom of ships, or being patronised by zoologists, is to make bloody certain humans don't find out about it."
Men At Arms
"Sham Harga had run a successful eatery for many years by always smiling, never extending credit, and realizing that most of his customers wanted meals properly balanced between the four food groups: sugar, starch, grease and burnt crunchy bits."
"Sometimes it's better to light a flamethrower than curse the darkness."
"Humans! They lived in a world where the grass continued to be green and the sun rose very day and flowers regularly turned into fruit, and what impresses them?"
"Its a popular fact that 90 percent of the brain is not used and, like most popular facts, it is wrong. Not even the most stupid Creator would go to the trouble of making the human head carry around several pounds of unnecessary grey goo if its only real purpose was, eg, to serve as a delicacy for certain remote tribesmen in unexplored valleys, it is used. One of its functions is to make the miraculous seem ordinary, and turn the unusual into the usual. Otherwise, human beings, forced with the daily wondrousness of everything, would go around wearing a stupid grin, saying "WOW" a lot. Part of the brain exists to stop this happening. It is very efficient, and can make people experience boredom in the middle of marvels."
"Racism was not a problem on the Discworld, because--what with trolls and dwarfs and so on : speciesism was more interesting. Black and white lived in perfect harmony and ganged up on green."
The Last Hero
"...frankly, the overlap between demons and gods was a bit uncertain at the best of times." "I believe all religions do reflect some aspect of an eternal truth, yes," said Carrot.
Lords and Ladies
"When I'm a witch I shall be able to go anywhere."
"But you'll never be a witch."
"They say you won't listen. They say you can't keep your temper. They say you have no discipline."
The girl tossed her hair. "Oh, you know that too, do you? Well, they would say that, wouldn't they? But I mean to be a witch whatever they say. You can find things out for yourself. You don't have to listen to a lot of daft old ladies who've never had a life."