Syndrome explaining that the 'S' on his costume stands for "Sitter."
Syndrome: "Originally, I was gonna have initials for "Baby-Sitter," but then I would've been going around wearing a big BS, and..." (laughs) "...you understand why I couldn't go with that."
Kari just... sitting there, having become, if not particularly accustomed to Jack-Jack, at least capable of dealing with whatever power manifests at a moment's notice, e.g. extinguishing him when he turns his flame on.
Dug's Special Mission
All of it, as Dug repeatedly acts as The Millstone for the rest of the pack. "Alpha, the rock moved!"
Alpha: Now, your mission is to sit on that rock.
Dug: Okay! (jumps to the other rock causing the rock Alpha, Beta, and Gamma are on to tilt. The three dogs begin running around trying to even the balance, but the rock falls and the dogs fall, screaming)
Dug:(jumping from rock to rock) Find the bird, find the bird! (Alpha, Beta, and Gamma follow, screaming as the rocks fall and Alpha goes down with one)
Alpha: Ow! I am hurting!
George and AJ
The two eponymous characters being in Jaw Drop mode for more than a full day!
The many ways the houses scram (Cats, slingshot, hovercraft, drill, oxygen rocket)
The commentary for it on the Pixar Shorts DVD has the director, after a minute of having no idea how to do the commentary, bringing in Corey Burton, using his Disney announcer voice, to do the commentating for him. Hilarity Ensues.
I'm sure everyone in the theater was in hysterics when the alien ship crashed onto the house unexpectedly. Subverted heartwarming ending, anyone?
A small one, but the little alien pushing his teacher out of the way, leaving a large handprint which starts slowly sucking back into shape like gelatin. If memory serves, Pixar built a new software simulation engine just for that gag.
Then there's the moment when the alien just completely gives up in frustration and starts hitting all the switches.
And the result of this action.
For The Birds
The tiny birds. Seriously, they all sound like squeaky toys.
When all the littler birds are mocking the big, goofy one when they first see him.
Just hearing all those silly sound effects made me laugh.
At the end of Day & Night, when Night has dawned upon Day, and he/she/it immediately realizes... hot sunbathing girl!
When the little boy is holding his papa and grandpa's brooms, and the broomheads both resemble their facial hair.
Mater and the Ghostlight
in the ending scene before the credits, the Sheriff warns Mater, who was already scared out of his wits over the non-existent Ghostlight, about the Screaming Banshee. After the credits are done rolling, we're treated to a scene of a monstrous truck (who is most likely the Screaming Banshee himself) looming over Mater. His reaction upon turning around? Warn the truck about a "Banshee" and wish him goodnight before leaving.
Mike's New Car
"It's like we've unleashed the panther."
The part where the car goes berserk and a guy walking down the street sees it and starts running in the other direction.
"Hey! Don't just sit there! Push the button!"
Sulley:(cell phone rings and he answers it) Hello?
The return of Spanish Buzz, and his lightstick fire dancing.
"We removed the poop from the kitty litter."
Chuckles the Clown leading the luau, reading his lines completely deadpan.
What about the part with Ken and Barbie pretending that they were deep-sea diving? The part with Ham even switching the tv that they were swimming in front of from a tranquil underwater scene with fish to a shark submerging out of the water, making Ken and Barbie freak out made me die laughing.
There's also the part with Mr. Pricklepants having the three Pizza Planet aliens audition for his latest production. Initially, he thought that the first two aliens' performances were mediocre or dreadful, but he liked the third one's great emotion. The cincher? All three aliens were uttering their trademark "Ooh" in the exact same tone.
And let's not forget at the end of the short where Ken and Barbie stepped outside in the snow in their winter clothes to share their first kiss before walking off the front step of the porch and into the deep spot of the snow. The scene after the credits even showed Woody and the others using a blow dryer to defrost Ken and Barbie from the ice block that they were frozen solid in.
Woody: Ride like the wind, Bullseye!"
Ken: Ride like the wind, Buttercup!
Buttercup: Okay, get off me.
The Bonnielikeoke motel.
Ken: I thought we were staying at a resort?
(Jessie puts up sitcky note with the word "resort" on it over the word "motel")
A little police toy car drives by, seemingly to control the situation. Then the police car dives into the water for the fun.
Just everything! It's all one big Mind Screw: Is there actually two of the same man? Does he have a split personality?
The way that Geri with glasses beams at Geri without glasses after winning the chess game.
What makes this even more Hilarious in Hindsight is that this may very well be the same old man who repairs Woody in Toy Story 2, and who even keeps his chess pieces in one of the drawers of his tool chest!