Airplane, What About Bob?, Dumb and Dumber, and Trainspotting–only in certain parts, of course!
Monty Python & The Holy Grail
South Park: Bigger, Longer, and Uncut
Ghostbusters
Zelig
Only one, no matter how many times I see it:
Blazing Saddles
Groundhog Day
LA Story
Ghostbusters
Quick Change
Office Space
Anchorman
Half-Baked*
Return of the Pink Panther
How High*
White Chicks**
Young Frankenstein
South Park: Bigger, Longer and Uncut
Airplane
Monty Python & the Holy Grail
I second dwc1970’s vote for just about all John Hughes movies.
My favorite movie, Pulp Fiction, contains a line that is, thanks to its delivery, comedy gold: John Travolta as Vincent Vega saying “Oh shit, I shot Marvin in the face!”
The way he says it in that situation is just priceless. Cracks me up every time.
*My state of mind during the times I’ve watched these movies probably contributes majorly to my enjoyment of their humor.
**I know, I know, but it’s really much better than you would think. (I was also on DXM at the time, which might have helped.)
Seconding:
Python & Holy Grail
Sleeper
Mr Blandings
This Is Spinal Tap
Ghostbusters
Waiting For Guffman
Best In Show
A Mighty Wind
Office Space
Airplane!
A Night At the Opera
South Park: Bigger, Longer, and Uncut
Swingers
Clerks
Mallrats
Pulp Fiction
Blazing Saddles
Billy Madison
Drunken Master (dubbed)
Orgazmo
Say Anything
High Fidelity
Even though it’s already been said several times, I have to echo Office Space; having worked in both cubicle farms and in a theme restaurant it was a bullseye as far as relativity.
Some that haven’t been mentioned:
Sordid Lives = a no-budget cult classic about a redneck family convening for Grandma’s funeral. The movie is completely stolen by Leslie Jordan as institutionalized transvestite Brother Boy. (Supposedly in San Diego this movie has been given the Rocky Horror treatment of midnight showings and simultaneous recitation of lines.)
Porky’s 2- it’s only one scene: Reverend Bubba Flavel & Coach Baubricker reciting and explicating Shakespeare’s Taming of the Shrew. Cracks me up every time.
Cat Ballou- Lee Marvin and the reclining horse make me watch this one whenever I find it while channel surfing.
Coming to America. There is absolutely nothing wrong with that movie. Loved it!
I’ll add another vote for Office Space, Airplane, Best in Show, and Some Like it Hot.
Also:
Trading Places
Happy Gilmore
A Fish Called Wanda
No, you’re not alone 
Airplane
Amazon Women On The Moon
Batman “Some days, you just can’t get rid of a bomb!”
Blazing Saddles
Caddyshack
Young Frankenstein
Mars Attacks!
Monty Python’s The Meaning Of Life
Monty Python & The Holy Grail
UHF “You get to drink from the FIRE HOSE!!!”
Roadie
I second:
Office Space
Monty Python and the Holy Grail
South Park: Bigger, Longer, and Uncut
Clerks
Galaxy Quest
And, add:
Ferris Bueller’s Day Off
Drop Dead Fred
McHale’s Navy
The Big Lebowski, Raising Arizona, and Airplane!
Shi mo’ fo buttah layin’ me to the bone! ‘S jackin’ me up! Tightly!
All of mine have already been said:
This Is Spinal Tap
Monty Python and the Holy Grail
The Emperor’s New Groove
Toy Story is close, but I think that’s more of a “geez, how brilliant and clever” movie than a “I can’t breathe because I’m laughing so hard” movie.
Office Space is awful.
I forgot the trilogy of awesome Eddie Murphy movies: Beverly Hills Cop, Trading Places, and Coming To America!
His stand-up film Delirious cracks me up too, but Raw, his other one, has aged really poorly.
A little background: Rarely, any film scenes make me laugh out loud. Others can be roaring, but I’ll be tight-lipped. Maybe it was because my folks hardly laughed at too many movies.
That said, there is a fight scene in The Human Tornado that always make me laugh SO hard, I cry. It’s a '70s movie with Rudy Ray Moore in his Dolemite character. The scene starts in daytime (cue: cheesy organ music) with Dolemite running around a house. Suddenly, it’s night and he’s jumped by goons and a speeded up fight/kung-fu scene ensues. At the climax, Dolemite throws a guy off an indoor balcony. The frame is stopped, with the guy in midair, reversed (back in Dolemite’s arms), forwarded, reversed, and forwarded again. The attacker lands on his back, eyes fixed and blood from his lip and Dolemite says, “You shoulda stayed down there the first time.”
I. die.
Also, many scenes in Fargo crack me up without fail.
Spies Like Us
Galaxy Quest
Holy Grail (Only the part where Lancelot slaughters the entire wedding party)
Me, Myself and Irene (the cut Hole-the-watermelon scene is gold!)
I go back and forth with Office Space. On one hand it’s really funny because, yes, as you said, it hits the bullseye as far as relativity. On the other hand, sometimes I watch the beginning and just cringe, because it hits so close to home. That bit with the TPS reports and the cover sheets just reminds me too much of my time in Office Hell.
In addition to my favorites that have already been mentioned here (Office Space, Dumb and Dumber, High Fidelity, Austin Powers), I give you:
Wayne’s World
Stealing Harvard (The whole Kyle and Steve thing kills me every time. I simply cannot help myself.)
Boy, most of mine hav been taken, but, just to vote:
What’s Up Doc? (I can’t believe how many of you picked this, too.)
Young Frankenstein
a Night at the Opera
Airplane!
Airplane Two (Yeah, I know it’s not by the same folks. But they did a good job)
Top Secret
Kemtucky Fried Movie
What’s up, Tiger Lily?
The Naked Gun (And the TV series Police Squad. Not so much the sequels)