Sherlock:
Sherlock’s skyping to John and a policeman who are at a crime scene. He’s talking about the chief witness, calling him obese, a basement dweller, has halitosis, etc., thereby eliminating him as a suspect:
“Does he seem like a master criminal to you?”
and then turns around to reveal the mortified said witness sitting behind him.
Mad About You, when Jerry Lewis plays a billionaire who hires Paul Riser to do a documentary on him. Steve Wright is the cameraman, and there’s a bit where Jerry tries to get Steve to eat a cookie, and he won’t open his mouth or say anything, so Jerry pushes the cookie against Steve’s mouth to try and force it in. When that fails, he tries to push in the crumbs. You can see Steve is struggling not to lose it.
Someone already mentioned it upthread, but *I have a youtube link. * 
Big Bang Theory where Penny gives Sheldon a napkin signed (and used) by Leonard Nimoy:
Sorry for totem-posting, but my edit time expired as I was typing!
Couldn’t find a clip for this one, but That 70’s Show. Kelso, Eric and Hyde are in the basement and Kelso’s talking about panties and “doing it” with Laurie, Eric’s sister. He starts rummaging through the laundry basket on the dryer looking for a pair of Laurie’s panties as a souvenir. He pulls out a pair, holds it up and Eric yells, "Hey man, those are my MOM’S!! I was howling.
The entire episode where Grandma Forman dies. 
Mystery Science Theater 3000- The Creeping Terror-The scene on Lovers Lane where the guy sits in the car for several minutes watching the Terror flip cars and eat people. He finally says “God”! and puts the car in gear to drive off. Mike says, “that’s the last straw for that guy”. Completely cracked me up.
The original Mr. Hankey episode of South Park.
South Parks Good Times With Weapons. Link.
The first 7 minutes or so, with the animation shifts and jokes, are pretty funny. Then around 7:45, Kenny chucks his ninja star at Professor Chaos and I about die every single time.
Definitely Mr. Bean’s Christmas (nativity scene,) the “Short Haired Judy Doll” episode of “Curb Your Enthusiasm” and the “Dwight’s Fire Drill” episode of “The Office” where the cat gets thrown through the ceiling and then falls back out on the other side of the room.
Pretty much any Night Court clip, but particularly one with Brent Spiner as Bob Wheeler.
One taste
You forgot the lines after!
Francis: Did it say when our vision would come back?
Reese: Box says two days.
Francis: Totally worth it.
From Roseanne: Jackie makes a phone call
My favorite from Fawlty Towers : The Talking Moose
Japanese, was it?
Another Night Court in the mini-opening after the credits and before they go to the first commercial. They gang is all sitting around a table, reading the paper. Someone says, “It says here they’re paying $50,000 for a vial of top-grade bull semen.” Heads around the table swivel towards Bull, who waits a beat, then says something along the lines of “Hey, I only give cash.”
It’s beautifully timed to coincide with our realization of the implication, and perfectly played
Many of my favorites are already listed. My favorite gut buster is the Reverend Jim and Driving license and Richard Simmons on Whose Line.
Sean Penn with the Church Lady on SNL.
Debbie Downer on SNL at Disney World.
Roseanne Roseannadanna complaining about tight jeans.
Jane you Ignorant Slut.
Family Guy - ‘my hairy aunt’
I don’t know that I’ve ever seen an entire episode of Everybody Loves Raymond, but I will say hubby and I (both of us teachers and parents of two young boys) were howling over the parent-teacher conference Raymond and his wife attended for their kindergarten-aged sons. Ray is paranoid and defensive, and at one point during the conference blurts out, “IS HE STUPID? YOU SHOULD TELL US IF HE’S STUPID!” Hubby and I quote that a LOT.
Episodes has made us laugh the most, hardest, had-to-rewind-to-rewatch-because-tears-are-pouring-out-our-eyes lately. Let’s see, there’s Matt LeBlanc talking about his penis size: “It’s like a baby arm. But without the hand.” Stephan Mangan, with his exquisite timing and semi-stunned deadpan: “I assumed.”
One that had me laughing hardest this season was Matt LeBlanc describing sex with a deaf woman. I was laughing through tears, then feeling like a massive asshole for laughing through tears over a deaf joke, then rewinding, rewatching, and laughing again.
Apparently I wasn’t alone–from USA Today online:
Found a link! http://www.youtube.com/watch?v=V6kzPQDRXo4
Oh shit, that is funny! I wish I had HBO (?) or whatever channel it’s on.
Father Ted: The “spider baby” scene in the carnival episode.
Mork and Mindy: The very first time I saw the Exidor episode with the Friends of Venus, and especially Mork’s FoV monologue. “Friends… I have need of yer atTENshun…”
Whose Line Is It Anyway?: The Richard Simmons skit.
Frasier: The one with Niles and the flour sack, where he describes his dream that someone kidnapped it and kept sending him muffins in the mail.
Several of the shorts on MST3K, especially the railroad safety ones (“My man is average…” “Obey the toaster…” “Trains are blameless, holy creatures…”)
Hometownboy:
Blood. “I only give BLOOD.”
South Park. The episode where the kids are in woodshop. Two kids run up to the shop teacher and one says something like “Mr. Adler! (that kid) got his face caught in the belt sander!” and (that kid) runs on screen, without any facial features at all. Kills me every time.
Since no one’s mentioned it yet, Strangers With Candy. The episode about steroids, near the end, was the longest continuous stretch of crying, gut-busting laughter that I can remember from a TV show. Sadly, I can’t find a video clip of it.
Also, most episodes of Arrested Development.
Family Guy - the ipecac/vomit episode