Many - some already mentioned.
One that immediately comes to mind:
Cheech Marin’s soliloquy in front of the bar in From Dusk Till Dawn. Those who have seen it will know what I’m talking about.
Pretty much the entire opening cartoon scene from Who Framed Roger Rabbit.
The “road trip” sequence of Animal House.
The cliff-jumping in Butch Cassidy and the Sundance Kid.
And hey, I liked What’s Up Doc, too!
In the no-accounting-for-taste section, the scene where Lassie gets turned on in Porkys never fails to dissolve Mrs. Kunilou into a puddle of tears.
The gangsta-style execution of the fax machine in Office Space.
Not exactly a movie, but the hearing aid scene from one of the Fawlty Tower’s episodes (can’t remember which offhand).
The scene from “Naked Gun” where OJ busts into the ship and pratfalls through increasingly improbable set-ups. (the wedding cake, the bear trap…)
I am laughing out loud now thinking about it 
It is definately the scene from Christmas Vacation where Chevy Chase cusses out his boss. No matter how many times I see that I crack up! It’s hysterical! Also the scene from Planes, Trains, and Automobiles where he is in the rental car line cussing out the woman the counter. The best part of that scene is the end: “‘Oh no.’ ‘What?’ ‘You’re f-----.’”
Just re- watched the Jack Benny/Carole Lombard TO BE OR NOT TO BE with my 10-year-old daughter yesterday. It’s just RIFE with delicious scenes.
The early one, where Benny (playing a Nazi) is telling little Wilhelm the Hitler joke: “They named a brandy after Napoleon…Bismarck became a herring…and our Fuhrer will…” And the other actor in the Nazi uniform says “…end up as a PIECE OF CHEESE!” And Benny glares at him and asks how he knows the punch line, and the guy says “Well…it’s an obvious thought!”
And any of the scenes with Benny playing Colonel Ehrhardt or Professor Siletsky, but especially the ones where he’s playing off the great character actor Sig Ruman.
The one where Ruman tells the Hitler joke and Benny yells “That’s not funny! And THE FUHRER vill not think it is funny, EITHER!” and Ruman’s eyes pop as he begs Benny not to rat on him. And of course when Benny shaves off the dead Siletsky’s beard and glues on a false one, and Ruman yanks it and it comes off in his hand.
Not to mention the first scene between Carole Lombard and Robert Stack, as the handsome young Polish bomber pilot.
Stack: “Do you know, I can drop three tons of dynamite in two minutes?!?”
Lombard, undressing him with her eyes, breathily: “Do tell…”
South Park Movie, the song “Uncle Fucka”
Snatch!!!
One of my favorites is the plaza scene toward the end of “Ruthless People” when the dumb guy shows up to pick up the ransom money. I think that was the hardest I have ever laughed at a movie scene. (I thought the whole movie was cleverly written.)
Another good (but juvenile) one was Frank Drebin in the bathroom when his cordless microphone started working again during a press conference in one of the “Naked Gun” movies.
“The Price is Wrong Bitch!” The whole fight scene cracks me up without fail.
“Don’t suck any Dick on the way to the Parking Lot! Hey you, get back here!”
This whole little scene from Swingers… “She’s like a little bunny. And you got these huge claws.” “And you’re just battin’ it around, you’re toying with her.”
I’m sure I’m forgetting stuff…
Top Secret
One of my personal favorites. The train sequence, the restaurant, the underwater bar fight… classic. Best Abrams/Zucker/Zucker flick out there, for my money.
UHF
Not high comedy, but some of the funniest moments ever. The answering machine message. The Wheel of Fish sequence. “I’m thinking of something… orange!”
Rosencrantz And Guildenstern Are Dead
Definitely high comedy. The paper airplane sequence. Questions. Coin-flipping determinism.
I’m off to buy some new DVDs…
Ah, you’ve reminded me of my favorite UHF moment – “SUPPLIES!” 
(Absolutely hilarious in context, absolutely meaningless to everyone else)
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The “no crying in baseball” scene in A LEAGE OF THEIR OWN
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The Black Knight from MONTY PYTHON AND THE HOLY GRAIL
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Several scenes from PULP FICTION: “Mother…fucker!”, the foot massage argument, Uma Thurman’s overdose, and Harvey Keitel’s scene as the cleaner.
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DR. STRANGELOVE when Peter Sellers as the president must call the Soviet premier and explain that they have accidentally launched a nuclear attack (“Um, Dimetri… Do you think you turn the music down a little?”)
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Riggs & Murtagh disarming a bomb in the beginning of LETHAL WEAPON III.
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In MONTY PYTHON’S LIFE OF BRIAN when the Roman soldiers correct Graham Chapman’s latin grammar as he vandalizes a Roman wall. Also the “Biggus Diccus” scene!
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Most of THIS IS SPINAL TAP, but especially the Stonehenge song, and when they get lost in the bowels of an auditorium.
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The stuttering scene in A FISH CALLED WANDA.
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A number of scenes in ANNIE HALL, including Marshal McCluhan’s cameo, dinner with Annie’s parents, and when their thoughts are subtitled.
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In LIFE IS BEAUTIFUL when Roberto Bernigni is translating the German guard’s orders into game instructions for his son.
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INDIANA JONES AND THE LAST CRUSADE: “Don’t call me Junior!!!”
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First 15 minutes of THE GREAT ESCAPE, and the scene where James Coburn encounters the French Resistance.
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THE PRODUCERS: “Springtime for Hitler”.
Lots of others, and I look forward to reading what the rest of the Dopers post!
Good gawd almighty, how could we have gone this far without that scene being mentioned?!? One of the funniest scenes of all!
I’ve got to second “Springtime for Hitler”. I was amazed I got so far down the thread before reading it.
So far no one has mentioned the “make em laugh” scene from Singing in the Rain. The first time I saw that I was laying on the floor crying with amazed laughter. I still chuckle just thinking about how funny Donald O’Connor can be. And how amazingly athletic!
“Springtime for Hitler” of course, of course, of course.
One of the greatest comedies and no one mentioned it until you!! Grok.
(Sound of TV hitting his head on keyboard.)
Someone already mentioned that scene from Life of Brian, but I’d like to second, third, and fourth it. It is a masterpiece that has cracked me up on every viewing for nearly 20 years.
Also, every time Clouseau says anything at all in the Pink Panther movies. That accent just kills me!
I’m in rural Colorado, so probably everybody but me knows. Did you hear that “The Producers” is on its way to Broadway? I caught a mention on Salon. It’s staring Nathan Lane in the Zero Mostel role, Mathrew Broderick in the Gene Wilder role, with Mel Brooks doing some bits.
I understand there is a dueling tap piece between Hitler, Churchill, Stalin and Uncle Sam that is wonderful.
“Noises Off”…the entire movie has me nearly wetting myself. It’s funny anyhow, but if you’ve EVER done ANY live theater it is screamingly funny.
“Kuffs”…a little-known, little-watched Christian Slater vehicle with Tony Goldwyn. There’s a scene wherein they’ve just almost been hit by a bus. Tony Goldsyn starts yelling at Christian Slater, and they bleep all of the “fuck” words…you know, “fuck,” “fucking,” etc…with different sound effects. Your standard “boop,” but also “boi-oi-oing,” “<guitar chord>”, etc. This is amusing in that it takes one by surprise on first viewing. The kicker though, is after this huge rant, “If I die on eht job, it’ll be because of a #%@#@^% bullet, not a @#$%@# bus!” Christian Slater makes some wholly inappropriate for the mood comment, and Tony Goldwyn says, “Ffffffffffffuck you.” The only one they didn’t bleep.
Okay, it sounds really stupid typed out. I mean, REALLY stupid. But when I watch it, it still gets me.
Same movie, Christian has just had his entire apartment shot up, he and a Bad Guy trying to kill each other. Bad Guy’s lying dead in his kitchen. There’s a knock on the door. He opens it and shoves his gun in the faces of the guys on the other side…the police chief and a guy who’s auditing Christian for fitness to serve. The door opens, gun comes up, and the investigator just goes, “PLLLLLLLLLLLBTH!!!” and keels directly over to the side. Again, it looks like nothing in print, but in the movie it kills.
All of “Dogma.” Holy cow. Especially, “If I had a dick I’d go get laid.” “We can do the next best thing.” “What’s that?” “Kill people.” <lady chokes> “Oh, not YOU.”
Man, this post makes me look LAME LAME LAME. Shuck-ee-darn.
Circus Scene in Fear and Loathing in Las Vegas after they have taken the ether.
Also in same movie, the begining, driving through the desert. Come to think of it the ends pretty funny too, as is the middle.
Mallrats, where Jay and Silent Bob are trying to destroy the stage. Cracks me up.