That one “insanity test” flash page where you see a picture of a motorcycle and a sped up audio track of a person making the sound of a revving motorcycle engine usually gets me.
The Daily Show these days. And not Jack Black but LEWIS Black. I *love * that guy. He has me on the floor. Same with Ed Helm’s Digital Watch.
The Games (an obscure Australian sitcom about the making of the Olympic games there).
Lucky Jim by Kingsley Amis.
Surprisingly there were a lot of moments in Homicide: Life on the Streets that totally cracked me up.
Saturday Night Lives:
The Whole William Shatner episode
The Dean Martin/Carl Sagan Christmas Special
Dana Carvey as Tom Brokaw reading all possible upcoming news stories so he can spend the winter on vacation. (it was senseless)
And the obvious:
Monty Python Include A Fish Called Wanda when it came out (asshole!)
South Park - the early years. The MOVIE.
MST3K
The Hitchhiker’s Guide to the Galaxy.
and Cecil often cracks me up. So do a lot of the posts here.
Lewis Black
That is all I have to say. Well, I said this but it doesn’t count.
I can’t believe I left out (of my OP): 70s - Woody Allen.
80s - Robin Williams
90s - South Park
00s - Daily Show
Not quite making the cut: Monty Python, Friends, Cheers, Seinfeld.
Weird Al Yankovic videos. All of them. The funniest is Like a Surgeon. I’ve seen it a hundred times, and I still laugh out loud when the heart slips out of his hands and shoots across the room.
The song “Fred” by Rodney Carrington.
Most of the music put out by Bob Rivers.
The following movies:
Clerks
Office Space
U.H.F.
Raising Arizona
Shanghai Noon
‘Fargo’
‘Raising Arizona’
‘Stripes’
‘Pulp Fiction’
‘Ferris Bueller’s Day Off’
Samuel L. Jackson in ‘Die hard With a Vengeance’
Marx Brothers
Bill Murray [who isn’t just a great comedian, but an excellent actor as well and ought to get an OSCAR. grrrr. ]
MAS*H
All in the Family
Monty Python
Fawlty Towers
Absolutely Fabulous
All books by Carter Dickson.
All books by Marten Toonder.
All books by Craig Rice.
Some of these have already been noted, but I have to cast my vote for them too…
Terry Pratchett
Eddie Izzard
Robin Williams–both stand up and movies. Hell, even his voices for cartoons are hilarious! “Gravity works!”
One-liners from Army Of Darkness (“This is my BOOM-stick!” "I may be bad , but I feel good! ")
Ghostbusters, Good Morning Vietnam, History of the World (Pt 1), MP’s Holy Grail
Calvin & Hobbes
Foxtrot
Patrick McManus (*A Fine And Pleasant Misery, Never Sniff A Gift Fish, * etc.)
Dave Barry in his prime cracked me up.
I like South Park pretty well, but I absolutely screamed during the film.
Mad Magazine when it was really good (and I’m an adult now, so by definition Mad Magazine was MUCH MUCH MUCH better when I was a kid than it is now. Thus has it been for every generation and thus shall it ever be, world without end. Amen.)
The Daily Show cracks me up quite regularly.
Comedian Jeremy Hotz had me rolling during his live show.
But the thing within the last couple of years that had me laughing so hard I could barely breathe, is James Lilek’s book, “The Gallery of Regrettable Food.” I am inexplicably incapacitated any time I read even a few pages.
I would find Will Ferell funny if he had a spear sticking through his head. Then I’d howl, otherwise… yeacch!
Ben Stiller is hit or miss. “Flirtiing with Disaster” is one of my absolute favorite comedies, and I really liked “Something About Mary” as well, but he’s done a whole lot of sub-par work.
And don’t even get me started on Adam Sandler. Painfully unfunny.
Jack Black in Bob Roberts: Instensely and frighteningly realistic. Great!
Jack Black in High Fidelity: Howlingly funny from beginning to end.
Jack Black in School of Rock: Quite good.
Jack Black every other time I’ve seen him: Not particularly funny, and a little unnerving, like watching an out of control dog without a leash, even in parts of School of Rock, especially in the DVD extras.
Now:
The Coen Brothers
Dr. Strangelove
South Park
Vince Vaughn
The Daily Show
Seinfeld
Scrubs
Arrested Development
Then:
Far Side
Mad Magazine
The Simpsons
Tiny Toons
Do you mean this?
Cracks me up every time.
I love the Marx Brothers. Groucho can have me laughing 'til I cry. “Why don’t you bore a hole in yourself and let the sap run out?” 
MST3K
The Simpsons (especially around seasons 3-6)
Kids in the Hall
The Far Side
WC Fields
Monty Python
The Young Ones
Red Dwarf
ACK!!! I almost forgot Arrested Development!!! What was I thinking?
Howard Stern when it’s just the gang sitting around chatting is pretty funny - not when there are strippers and porn stars there. Also, when they’re talking about India or Pakistan and they play that recording of The Fifth Dimension’s Up, Up and Away played on the sitar. That kills me everytime.
I am also a big fan of Family Guy.
Anybody read Lucky Jim by Kingsley Amis? It’s a bit dated now (1956) and veddy british, but god is it funny. I mean tears streaming down my face, barely able to breathe funny.
In fact I think it may be the funniest thing ever. IMO, of course.
(on preview, I note uglybeech already mentioned the book. But it’s so funny I think it deserves a post of it’s own.)
I haven’t read a Mad in 30 years, and I’d be curious to see how much of the 60s humor holds up, or how much of what was funny to a 10 year old is funny to a 50 year old. On second thought, I’d rather not know. But when George Woodbridge died recently, and a link was posted here to 43-Man-Squamish: man, still funny.
Yes!
Funniest thing I’ve seen in years is one scene in Dodgeball - the ESPN 8 Intro. “Super Bowl, World Cup and WWII combined…if it’s almost a sport, it’s on The Ocho.” Laughed til I cried.
The Smothers Brothers. My parents had several of their albums, and when I was a little kid, their rendition of “The Fox” had me rolling on the floor laughing. And over the years, that and many of their other routines remain laugh-out-loud funny.
The Onion. Every once in a while I’ll read something in there that hits me just right and I’ll be struck with the giggles.
Many others that have been mentioned can get me to laugh out loud if I’m in the right mood and/or with the right people: SNL at its best, Monty Python, The Young Ones, MAD, Dave Barry, South Park, Simpsons, Family Guy, Conan, Weird Al’s videos, Bill Cosby, Calvin & Hobbes, Peanuts. Warner Brothers cartoons
SNL used to crack me up, but now it sucks. I’ve turned to MadTV. Ron Pedersons Clay AIken cracks me up. Aries Spears Shaq does it too. Frank Caliendo’s impersonations are spot on, and his President Bush is better than Will Farrells though “Strategery” still gets me every time.
Everyone else on the new Saturday Night Live needs to burn in the hottest level of hell for all eternity.
Bob and Tom and Car Talk are the radio programs that never fail. I only wish that my local NPR station still carried Car Talk. I miss Click and Clack so much. 
I used to think Weird Al was the shizzle, and he still is funny, but not as much as he was. His video for the song “Fat” is still scary funny. Now I like Tom Lehrer and Stephen Lynch.
Dave Barry is still funny, and his novels are funnier than his columns ever were. The dude is talented.
Early In Living Color. Jim Carreys Vanilla Ice skit can still send me to the floor. The part in the beginning where he says “Let’s kick it.” and kicks his shoe straight up and off. “Yo. I gotta learn how to tie a bow.”
Oh, who else. George Carlin, Steve Marting, Bill Murray, Hal Sparks, Lewis Black, The Marx Brothers, Looney Toons, Ed Edd and Eddy, Fairly OddParents, MST3K, The Daily Show, The Simpsons, Arrested Development, Malcolm in the Middle, Seinfeld (comedian and show), South Park, Airplane and Hot Shots, Mel Brooks, Denis Leary, Bill Cosby, Chris Rock, Kevin Smith’s movies and Jay and Silent Bob.
Of special note is Eddie Izzard who has made me laugh all of the way through his specials.
Almost forgot VG Cats. Shame on me. I check the site every Monday obessively for the new comic. If you’ve ever played a video game, you need to read his comics. Scott Ramsoomair is pure genius.
And lastly, my fellow dopers. There are too many truly funny posts and threads to list here. Ya’ll crack me up on a daily basis.
so I’m a kiss-up
You’ve been one of my more favorite posters the past month and a half or so but I think I might have to start actively avoiding any thread you participate in now. Bob and Tom? I knew that they had to have fans because they’re nationally syndicated but I always hoped that Dopers wouldn’t’ve been included in their fan base.
Today is a very sad day.
Douglas Adams’ Long Dark Tea Time Of The Soul.
It’s my favorite of his books. I laugh out loud every time I read it.
Chimps and guys in cervical colars.
Alias, I forgot W.C. Fields. Thanks for the reminder. 
“What a gorgeous day. What effulgent sunshine. It was a day of this sort the McGillicuddy brothers murdered their mother with an ax.”
Oh, and Dorothy Parker. *
“It serves me right for putting all my eggs in one bastard.”*