IT/Nightmare Fuel

Ah, IT. Regarded as one of Stephen King's scariest (and longest) novels, it sure does have Nightmare Fuel. Now, where are my brown pants?

Then it was adapted into a movie, and untold numbers of children suddenly were scared of clowns. Good reason, too.

The Movie Adaptation

The scene where the sink starts spewing blood is horrifying and disgusting, and made even worse by the fact that the Beverly's dad doesn't even see the blood.
The shower scene in which Pennywise is shown calling out to Eddie from below one of the shower drains, lifts open the drain, sticks his hands out and basically shoves at the drain opening with his hands literally like it was clay to make it big enough so he can stick half of his body out and tell a horrified Eddie "Here I am, wheezy! Hey, you're gonna like it down here. Won't do any good to run, girly boy! See you in your dreams! Oh, come back anytime! Bring your friends!" No doubt it left kids with a paranoia fueled fear towards shower drains or probably drains of any kind.
- Pennywise then abruptly lowers his head and lifts it, letting out an loud unearthly roar before chuckling and looking directly at the camera while some creepy music plays.
Five Words: Pennywise in the storm drain.
"Kill? <chuckle> ME? Oh, you are priceless, brat. I... am eternal, child. I am the Eater of Worlds - and of children. And YOU... are NEXT!"
Akin to the scene where blood comes out of the sink is the scene wherein Bill is looking at an old photo album of his dead brother George, and suddenly Georgie's picture winks at him. He throws it across the room and the album starts moving on its own, turning back to that same picture, which then starts to bleed heavily. Then when his parents come in and can't see the blood.
The scene involving the sepia-tone photograph of Derry, with Pennywise carousing about in the photo itself, including running up to the picture, screaming at the kids, and reaching his hand out trying to grab at them.
This line from Pennywise: "I'll kill you all! *laughs* I'll drive you crazy... and then I'LL KILL YOU ALL! I'm every nightmare you've ever had, I am your WORST DREAM COME TRUE! I'm everything you EVER were afraid of!"
Stan's suicide.
Tim Curry, in all that clown make-up as Pennywise. Even when he's not doing anything particularly frightening in the movie, he's still creepy as all hell. To give a good idea as to how creepy Pennywise is, the other actors/crew-members actually avoided Tim Curry while he in make-up and on set.
The scene where Ben has just confessed his love for Beverly and they kiss and embrace, only for him to look in the mirror and see that the person he is holding... is wearing a clown suit.
Beverly's encounter with Mrs. Kersh.
Eddie's encounter with Mr. Keene, and the idea that Keene's memory of Eddie was all fabricated by the Clown.
The idea that the adults can't see the blood and therefore cannot remove the blood until they wash the body part bloodied, means they could go up to a day without washing it off if its in their faces, and hours if its on their hands...there could be phantom alien paint all over Bill Denborough's and Bev Marsh's houses.

The Novel

One thing that stands out to many though is the dead boys in the water tower. Just the thought of falling into the water and having no way of getting out, just having to wait for death... Creepy beyond words.
How about the fact that Pennywise isn't confined simply to his own novel? He appears in several other Stephen King novels, and plays no direct role. He simply sits in the background, possibly waiting for something.
- Dreamcatcher. "Pennywise lives!" Oh, Jesus...
  - 1958+27=1985 and then 1985+27=2012. If Pennywise is still alive, next year promise to be very interesting for the people of Derry ... and for Stephen King's readers...
Pennywise in the storm drain.

"Everything down here floats."

Patrick Hockstetter. An insane and unfathomably sadistic solipsist who believes that he was the only "real" being in the universe who had no sense of hurting or even being hurt (even after Henry struck him in the mouth when he took a step further from a handjob and offered him oral sex).
- He suffocated his baby brother because he believed he threatened his existence without a smidge of regret or even worry of the possible repercussions that would follow if he were caught (which his father almost did when he found a pair of tracks from Patrick's boots towards the infant's room, but shut the thought out forever out of horror).
  - The description of him when he was five-years-old suffocating his baby brother. "He had a hard-on while doing it. This was not uncommon."
- He uses animals for his "experiment" where he tortures them ever so slowly by placing them in a rusty refrigerator to freeze and starve, including a stolen cocker spaniel puppy whose torture lasted the longest.
- Said boy's fate, in chapter 17. He was a sociopath with only one fear: leeches. It takes the form of leeches combined with mosquitoes and wasps. The description of the boy being drained of his blood as well as the fact that one of the leeches sucks his eyeball dry is pretty horrifying as well. Even after ripping off and squashing a bloated leech feeding off his arm, the severed head was still embedded and continued to suck out the blood even as it was spilling out the other side like a hose. He was still alive while Pennywise was dragging his bloodied body into the sewer. He woke while he was feeding on him.
  - The audio telling of Patrick's death.
What happened to Michael "Mike" Hanlon when he was a baby. While his mother was cleaning and hanging laundry outside, Mike was sitting and playing in his baby carrier when a large crow showed up and started pecking his face. It wasn't until his mother heard his cries of terror and pain that she rescued and tried to kill the crow, but it already flew away. The fact that Mike was only a baby and couldn't defend himself is the scariest thing that could ever happen in his childhood. The crow appears later as a giant, horrible monster by one of Pennywise's manifestations.
- This troper has read many a Stephen King novels and nothing scared him more then the scene where Mike was hiding from Its giant bird rendition.
Pennywise appearing as the main character's little brother the day he died with his arm ripped off. Fortunately, the main character regains his senses and kicks Eldritch Abomination ass.
- For that matter, the scene where George got his arm ripped out. Especially:

George reached.
The clown seized his arm.
And George saw the clown's face change.
What he saw then was terrible enough to make his worst imaginings of the thing in the cellar seem like sweet dreams; what he saw destroyed his sanity in one clawing stroke.

What happened to Tom Rogan when he gazed into Pennywise's "dead lights". Since he was an abusive dick, he deserved having his head explode from madness.
The Deus Sex Machina. It's just so disturbing and horrible (despite being well-written, of course) that any inkling of Fetish Fuel is drained away into the bowels of Pennywise's sewer, never to be seen again. as the children fleeing the sewers, from defeating IT the first time, stop for a moment to make an entire connection via their reproductive organs.
Instead of appearing in the school shower to frighten Eddie like in the miniseries, IT harasses him in the form of a homeless man offering oral sex. King's description of the homeless man and his proposition is incredibly nauseating nevertheless. The form that the monster takes afterwards based on the fear of the homeless man; the homeless man had leprosy, so the form IT takes looks pretty nauseating.
The boys being chased by IT at the Neibolt Street house porch.
"Sometimes I worry about you Beverly. Sometimes I worry a lot." It's up in the air whether it's creepier coming from her own father or from IT disguised as her father.
The underlying message the novel carried throughout its entirety is rather like a punch to the gut. The idea that such great chunks of our lives end up being forgotten, lost during the transition to and through adulthood.
Beverly's encounter with Mrs. Kersh. Especially when Beverly finds her cup filled with liquid shit.
Adrian Mellon's death. Beaten to a pulp, dropped off a bridge and when his lover goes to look to see if he's okay, he sees Pennywise biting into the man's body, before turning his head to look at the guy.
- Imagine growing up in one of the towns that King amalgamated into Derry. The murder on the bridge was based on real events in Lewiston, Maine. Near as anyone can tell, the water tower nearest there was in Norway, Maine. That book was all sorts of creepy for people native to Maine.
The fact that, when they went back as adults, IT WAS FUCKING PREGNANT! And they don't know if they got all the eggs! Oh, and what about that thing with the Ritual of CHUD? You know, where Bill has to BITE IT'S METAPHYSICAL TONGUE, which is described as all gross and cracked and scabbed?
- How did it get pregnant anyway? Fridge Horror indeed...
"And although the wall itself towered hundreds of feet above them, the door was very small. It was no more than three feet high, a door of the sort you might see in a fairytale book, made of stout oaken boards bound with iron strips in an X-pattern. It was, they realized at once, a door made only for children. Ghostly, in his mind, Ben heard the librarian reading to the little ones: Who is that trip-trapping upon my bridge? The children lean forward, all the old fascination glistening in their eyes: will the monster be bested...or will it feed? There was a mark on the door, and heaped at its foot was a pile of bones. Small bones. The bones of God alone knew how many children. They had come to the place of It."
- "Bill toed the bones, and suddenly scattered them in a powdery, rattling drift with one foot. He was scared, too...but there was George to consider. Were those small and fragile bones among them? Yes, of course they were. They were here for the owners of those bones, George and all the others—those who had been brought here, those who might be brought here, those who had been left in other places simply to rot."
The horror and destruction IT wrought whenever it awoke from its decades long hibernation. The beginning of its reign of terror in Derry, Maine in the 18th century starts only two words - "It Awoke". And of course It Gets Worse from there, especially the incident in 1906 when 88 children were gruesomely killed during an Easter Egg Hunt caused by an explosion from Kitchener Ironworks right before IT went to hibernation again.
The unfortunate destruction of the African-American military night club which was set fire by a group of "Legion of White Decency", in effect the KKK. One of the survivors, Dick Halloran, would become the cook for the Overlook Hotel. Mike Hanlon's father, Will Hanlon tells his son that on the night the club burned down in 1930 he witnessed giant bird carry off a LWD member in its talons—the same bird that nearly killed Mike in 1958 when he was an infant!
Imagine that the man you've married abuses you to the point that you mentally and emotionally regress to a child when you're around him. Welcome to Bev's life being married to Tom Regan until she has the guts to stand up to him and gives him a taste of his own medicine.
The realization that the kids had that the entire town was in a sense, in league with IT, especially when they realize that even their own parents weren't going to stop IT from taking them. It's bad enough as a kid to have to fight the boogey monster, but far worse to come to the conclusion that your parents are on the monster's side...
Henry Bowers carved the first letter of his name into Ben Hanscomb's belly with a knife, and would have put his whole name down there if Ben hadn't fought back and gotten away. The scar faded away after Ben left Derry, and then reappeared as IT began to reawaken.
The fact that Beverly's father was implied to be sexually attracted to her and abused her because of it.
Stanley Uris, one of the protagonists, commits suicide in his bathtub because he doesn't want to go back to Derry to face IT again. When his wife finds him, she finds that he's written the single word "IT" on the bathroom wall in his own blood.
Pretty much all of these examples and others become even more frightening upon listening to Steven Weber's reading of the novel. His matter-of-fact narration of horrific scenes leaves the listener no quarter, and the different voices he affects for Pennywise's different manifestations are particularly chilling.

Down here, we all float.