Two spiders standing on a leaf. One spider removes a paper bag from his head with a horrible face drawn on it, and says, “Did I scare you?”
The other spider has a long line of web hanging from his butt, lying in a pile below the leaf.
Damn you, big_yellow_kingswood!!!
BWA HA HA HA HA!!!
A well dressed, heavily made-up female cow is standing next to a window with a martini in her hand (hoof). She looks over at her husband cow who has a nose-ring, a beer gut and a tinnie watching TV. She says simply, “Wendell… I’m not content.”
I got really mad trying to think of my fav (or at least a good one), but instead I could only come up with a B. Kliban cartoon, which is kinda along the lines of Far side…
There’s this guy yelling at a Parakeet, a cat, a goldfish, and a really mean looking demon. next to them is a bed with human bones and organs lying around on it. The caption is “Harry, Louie, Curly, Ted, Who’s Been Eating In My Bed?”
My favorite was the Holsteins visit the Grand Canyon. I didn’t get it until Gary Larson pointed out on a talk show, that the little boy cow is doing “the old hoof behind the head” gag, to his sister cow.
I can’t believe no one else but me loves this one. I used to have it on a postcard, but can’t find it, so I’ll paraphrase.
Split panel: One side is a guy lying in bed thinking; I wonder if she likes me? I think she does. It’s hard to tell, though. She MIGHT like me. What if she doesn’t? I really like her…
Other side: Girl lying in bed thinking; You know, I really like vanilla.
Oh, god. Can you sum up my romantic life any better?
Okay, let me try again.
What we say to dogs: Bad Ginger. You’ve been a very bad dog, Ginger. I told you not to do that, Ginger. You’re in big trouble now, Ginger…
What the dogs hear: Blah blah Ginger blah blah blah Ginger blah blah blah blah Ginger blah blah blah Ginger…
In a later comic, what we say to cats: Bad Fluffy. You’ve been a very bad cat, Fluffy. I told you not to do that, Fluffy. You’re in big trouble now, Fluffy…
What the cats hear: Blah blah blah blah blah blah blah blah blah blah blah blah blah blah blah…
[hijack]
Ever notice how you can point to something and a dog will try to figure out what you’re pointing at, but a cat will sit there looking at your finger?
[/hijack]
I’ll second the motion of St Atilla.
Darn it all, big_yellow_kingswood that was the one (the two spiders)that I was going to say was my favorite.
Dang, now I have to come up with another.
Snake sitting comfortably in a lounge chair, halfway through swallowing a whole pig. Phone rings. Snake thinks “Damn!”
Had this pasted to my phone at work for the longest time.
Other one we used at work: Burly guy standing outside a restaurant, pointing at a sign in the window as two guys slink away. Sign says “No brains, No service.” We kept this pasted to the door of the programmers’ office in my old job, for when some user decided to disturb our Deep Thought.
My favorites were any that had the anthropologist characters in it. I swear, Larson followed me around in my dealings with different archaeologists/anthropologists because he drew them dead on.
But one favorite was the one where it shows a group of tribal people wearing bags over their heads, and the caton is like “Look Jenkins! It’s the legendary ‘ugliest people on Earth!’” or something.
http://boards.straightdope.com/sdmb/showthread.php?threadid=19145
There are so many to choose from. One of them shows one chimp grooming another, and the grooming chimp says, “A blonde hair? So, been conducting some more ‘research’ with that Jane Goodall tramp?”
Dr. Goodall saw this, guffawed, and said, “Hey, I’ve really made it! I’m in the comics!”
Unbeknownst to her, the apparently humorless director (a woman) of Goodall’s organization wrote a nasty letter to Larson which went something like “How dare you even suggest she had sex with the animals!” and so forth including threats of court action.
Some time later, National Geographic wanted to put this cartoon on a t-shirt. Larson’s people said, “No, we don’t think so,” and told them about the letter. Someone from Natl. Geo. said, “That doesn’t sound like the Jane Goodall I know” and contacted her. She was very surprised to hear about this and straightened the whole thing out. (The woman director had left the organization in the interim.)
Many which never made it into the strip were published in “The Prehistory of the Far Side.” One showed two spiders sitting on a blade of grass. One has a tiny paper bag over his head like a mask, and the other has a strand of silk from his abdomen down to a pile of silk on the ground. The one in the mask says, “Guess I really startled you, huh, Bob?”
Sorry, I should have read the whole thread before repeating the spider one. 
Two of my faves:
The Crisis Clinic. There’s a building in flames, going over a waterfall, and there’s a sign on the front of the building that says “Crisis Clinic.” My mom used to work at one of these. Every single staff member there had a mug with that on it.
My other favorite is one with two cavemen sitting near a fire. One of them is holding a piece of meat in the fire, and he has a very pained look on his face, because the fire is burning his hand. The other caveman is pointing to a third caveman, at another fire. The third caveman is holding his piece of meat over the fire with a stick. The caption reads “Ooh! Ooh! Look what Og do!” Reminds me of some of the people I work with. 
The aliens flying their saucer down a busy city street, scattering terrified earthlings before them, and yelling, “Yeee-haaa!”
Guy looking at a busted piano seat that just landed on the sidewalk in front of him, puzzled look on his face. Meantime, the piano itself is on its way down, maybe five feet from hitting him right on the head.
Okay, I’ve got three:
- February 15, 2000: It’s a medieval battle scene, with the knights on horses and their huge swords across from each other in a field, and then there are the two guys who are holding the reins. The one guy is holding his hand up to his face and his mouth is wide open, and the knight is brushing at his eye.
The caption: "Time out, please! … Eyelash!"
(No one else thinks it’s funny, so it’s okay if you don’t either.)
- Sometime in 1998: There’s a boy walking down the street with his dad, and a monster sitting there with a sign that says, “Nightmareless.” The boy, who has some unseemly hairs and big ole’ lips, is looking at the monster.
The caption: "Not heeding his father’s advice to avoid eye contact, Joey makes a ‘contribution’."
- Way back on April 30th, 1997: The cute dog has put his front paws on the coffee table and is eating out of a box. Through the doorway, the scarecrow is talking on the phone.
The caption: "Yeah, I just got back! And the wizard I mentioned? He gave me a new brain! … It’s on the coffee table as we speak!"
I’ve got the one-a-day calendars dating back to 1995; I have hundreds of others stashed away, but those three continually crack me up. 
On the chalkboard:
“kay pas-uh
aw blah es span yol
be-in fayo
bwayno dee-us”
Caption: Matthews - we’re getting another one of those strange ‘aw blaw es span yol’ sounds. I love Larsen.
Some of my favorites (beyond what have already been mentioned):
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God, in the kitchen, with a box of “Earthquick” and a hot Earth in a cake pan. He says, “Something tells me this thing’s only half-baked.” Caption: In God’s kitchen.
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God, as a kid, playing with a chemistry kit. It blows up, leaving him with soot all over his face and feathers flying around the room. Caption: God as a kid tries to make a chicken in his room.
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Rush hour, cars zooming by. Prehistoric man is standing off to the side of the road by the burned-out mechanical hull of a mammoth. Caption: Tantor burns up on I-90.
And so many more.
Some that haven’t (amazingly, given the number of posts in the two threads) been mentioned:
There’s a den full of snakes, and one is saying to another, “This place gives me the creeps!”
A pilot of a jumbo jet is saying to his copilot, “The fuel light’s on, Frank! We’re all going to die!..Wait, wait…Oh, my mistake - that’s the intercom light.”
And one whose caption reads “Carl Sagan as a kid”: A boy and a girl standing on a hill looking at the night sky, and he’s saying, “Just look at all those stars, Becky…there must be hundreds of them!”
Just remembered:
Scene from the back of a cockpit, looking out the front. There’s a small break in the clouds, with a mountain goat in the middle of it.
Pilot to copilot: What’s a mountain goat doing in up here in the clouds?
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Two cavemen with bows are looking at the gigantic mammoth lying belly-up dead, with a tiny little arrow sticking out of it’s stomach. “Wow, we better remember that spot!”
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In the background, a giant postman is on the rampage tearing down skyscrapers, while in the foreground a pack of dogs is being addressed by it’s leader: “It’s up to us to save the city!”
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A poker-playing cowboy angrily draws his gun. “Varmits! Yer all a buncha cheatin’ varmits!” he says to the squirrels, rabbits and gophers he was playing with.
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A dog tries subconcious suggestion on it’s sleeping owner: “Puuuuut the caaaat ooouuut. Puuuuuut the caaaaat oooouuuut”.
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The neighborhood children run and hide when they hear the bells of the “liver and onions” wagon.
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An astronaut on the moon reads a note left on the lunar module: “Where were you!? We waited and waited, but we finally had to…”