The bartender of The Falcon is really slow on the uptake realizing most of his customers were gay men, meaning his bar had unintentionally become a gay bar. In the end he decides he doesn't care because it's good for business and none of the gay men start fights.
Let's face it, just Richie, particularly in the book where his humor is allowed to get a lot more vulgar.
Richie: I had a friend whose balls swelled up to roughly the size of the tires on a 1959 Cadillac. I was gonna give him a pair of suspenders and a couple of barrels for his birthday - sort of a designer truss - but they went down before then.
Bill: All put with your customary tact and dignity.
Richie: Thank you, Bill, for those words of support. The word "fuck" was used two hundred and six times in your last book. I counted.
Gets even funnier in many translations of the book, since the word "fuck" in English can be either a curse word or refer to people having sex, while translators often have to choose which one to go with since it doesn't translate directly. Therefore you end up with several translations where Bill wrote about people having sex over two hundred times in his novel.
Just about any time Richie opens his mouth really.
Henry and his boys doing Fart Blasters, which got Beverly laughing despite being terrified out of her mind.
In the book, after Ben meets Bill and Eddie, Bill heads home. Eddie briefly talks to Ben about Bill and his stuttering problem is brought up. This after Bill spent a solid thirty seconds stuttering over the 'd' in 'dying.'
Eddie: Bill, he used to only stutter a little. Now it's really bad. Did you notice that he stutters ?
Ben: Well...a little.
Kay McCall, Bev's feminist friend, talking about her ex-husband, who she took to court for every cent she could get, despite it being seemingly against her feminist ideas:
Kay: Bullshit! The people who say that stuff never had to go to bed with Sam Chacowicz. Two pumps a tickle and a squirt, that was ole Sammy's motto. The only time he could keep it up for longer than seventy seconds was when he was pulling off in the tub. I didn't cheat him; I just took my combat pay retroactively.
In the novel, the story of Bill and Richie's sortie into the house on Neibolt Street bears some unexpected funny-fruit. Just when it looks like they can't escape the Werewolf's clutches, Richie takes some sneezing powder out of his pocket and blows it into the Werewolf's face. The way the author describes It's reaction to this is quite humorous, especially the look of stupid surprise on the monster's face.
IT was pregnant the whole time. Either she can reproduce asexually or some unrelated cosmic horror took some time out of it's day to blow Pennywise's back out.
In The Movie — "Excuse me, sir!! Do you have Prince Albert in a can?? You do?? Well, you better let the poor guy out!!" The incredibly lame joke isn't what's funny (and was also in the book), but Tim Curry's ridiculously hammy laugh.
"Kiss me, fat boy!" also deserves a special mention.
The scene in the movie where Bev first meets the losers club after 30 years and makes out with each one (Including the married Bill) is funny. Though those who didn't read or heard of the novel wouldn't get why but those who have would. Though it wasn't mentioned in the movie it is heavily implied that she had sex with all the members of the losers club. It's confirmed in the novel.
Stan's disembodied head in the freezer sounds like it'd be scary, but then the head starts talking and making jokes. "How's YOUR sex life? ... WHAT'S your sex life?"