> you forgot the one where springfield is having a feud with the neigboring
> town. someone mentions that the people in the next town over had threatened
> to spike springfield's water supply with lsd.

OK, OK... Let's get this over with, since there seems to be some interest.

There are innnumerable (illegal) drug references in the Simpsons, and since we all seem to be saying "yes, and remember the one where...", here is my list (you asked for it):

At the chili cook-off, Homer eats the guatemalan insanity chiles and trips, going on a sort of spirit quest. This one was just incredible, with little room for an alternate, "legitimate" interpretation. The visual effects are very impressive, and very obviously done by someone who has "been there". Ironically, it is Wiggum's chili that has the psychedelic peppers. (3F24)

In the bowling team / school uniforms episode, Smithers is giving Burns a manicure, and Burns is hopped up on ether for the pain:

Burns:	[groaning] Ohh! I need some more ether. I can still feel the movement of the emory board.
Smithers:	[checks bottle] We're fresh out, Sir. I'll get some more.
Burns:	Leave the rag. [sniffs it] Mmm... [Smithers walks out; Homer sneaks in]
Homer:	Uh...excuse me, Mr. Burns?
Burns:	[gasps] Poppin' Fresh! You glutinous little doughboy. [Homer morphs into Poppin' Fresh in Burns' mind] [chuckles] There's something I've wanted to do to you for years! [starts poking his stomach; Homer laughs]
Homer:	[getting his eyes poked at] Ow! Heh, Mr. Burns? I, er, was wondering if you'd like to sponsor my bowling team...for... $500.
Burns:	Ho ho, why, certainly, Poppin' Fresh! I owe my robust physique to your tubes of triple-bleached goo.
Homer:	Woohoo! [runs out] Hey, everybody: if you want to ask Burns for a favor, now's the time! He's doped up, or dying, or something.
Hans:	[running in] Uh, excuse me: I'd like to request $17 for a push-broom rebristling. [shows broom with no bristles on it]
Burns:	Why, it's that delightful TV leprechaun. I'm going to get your Lucky Charms. [starts a drill]
Hans:	Oh, no: my brains.

Later on, (non-colorfast) gray school uniforms turn all sorts of psychedelic colors when rained on, and Milhouse says "I'm freaking out!" (3F10)

Shelbyville spikes Springfield's water supply, in "just another chapter in the pointless rivalry between Springfield and Shelbyville." Marge drinks the water and trips. ("Oooh, the walls are melting." Turkey emerges from the oven, says "Personally, I think I'm overdone" and flies out the window.) (1F14)

In the episode where Bart becomes Mr. Burns' heir, there is a flashback to his youth, showing him as an undercover agent on a Greenpeace boat. When he's unmasked, he holds up a bong and says "And all this time, I've been smoking harmless tobacco!" (Very tongue-in-cheek on the part of the writers, IMHO). (1F16)

Bart and Milhouse get an all-syrup squishie. The manner in which they consume it is unmistakenly like smoking a bong. Then, to remove all doubt, Milhouse says "Don't Bogart that squishie." Barney happens by, tries some, and says "I don't know where you magic pixies came from...but I like your pixie drink!" (1F06)

Lisa drinks the water at a ride at Duff Gardens (supposed to be like "It's a Small World" at Disneyland) after Bart dares her. She trips. ("I can see the music." "I am the lizard queen." "Can't talk. Coming down.") (9F11)

At the beginning of the Be Sharps episode, Bart buys a bong at the swap meet, thinking it's a pencil holder. Homer says "Far out man. I haven't seen a bong in years." when Bart shows it to him during the car ride home. (9F21)

In the episode where child protective services takes Bart and Lisa away, Marge tests positive for crack and PCP after saying "It's like I'm high on some wonderful drug." When she retests clean, she says "The only thing I'm high on is Love for my Son and Daughters. Yes, a little LSD is all I need." (3F01)

When he runs for mayor, Sideshow Bob accuses Quimby of being an "illiterate, tax-cheating, wife-swapping, pot-smoking spendocrat". The scene shifts to Quimby, who is in his closet watering a pot plant. He says "Hey, I am no longer illiterate." (2F02)

When Milhouse becomes Fallout Boy to McBain's Radioactive Man, Lionel Hutz tries to become his agent, introducing himself as "Lionel Hutz, your new agent, bodyguard, unauthorized biographer, drug dealer..." (2F17)

In the episode where Bart gets an elephant, Homer is cleaning the basement, inhales the fumes of the various cleaning products, and trips out, thinking he's being attacked by various product logos. (1F15)

In the second episode with Homer's brother, Homer sits on the Spinemelter 2000 massage chair and trips (Actually, this is a 2001 reference, but it's damn like a trip anyway, and totally hilarious). (8F23)

In the union/dental plan episode, when Lisa gets braces, the dentist puts her under with N₂O. A Yellow Submarine-inspired trip sequence follows. At the end, the dentist forgets to turn off the gas, and he, Lisa, Homer, and Marge all laugh for a little too long... This same episode starts with Homer and Bart watching a really bad McBain movie. He bursts out of a Venus de Milo ice sculpture far too small for him that is the centerpiece of a gathering of some evil dudes whose plotting concerns a drug called "swank," which is "ten times as addictive as marijuana." (Very tongue-in-cheek) (9F15)

In the anti-foreigner episode, Barney drinks the drug from a tranquilizer dart. (3F20)

In the Whacking Day episode, Barney hallucinates and thinks snakes are attacking him. (This is DTs, probably) (9F18)

In the Homerpalooza episode, Cypress Hill orders the London Philharmonic while high. Bart says that Cypress Hill "smells like Otto's jacket." Homer gets accused of being a narc. Otto's shoes talk to him ("Don't worry. We won't hurt you. We only want to have some fun."). (3F21)

At the Spinal Tap concert, Bart is next to some guy who is obviously zonked to the eyeballs on something. (8F21)

Krusty is addicted to percodan. (9F07, 2F32)

Homer eats a packet of powdered gravy and hallucinates, seeing his sexy new coworker Mindy looking like Botticelli's Birth of Venus. Lenny and Carl fly by as cupids, and comment "Homer, what's the matter? Ain't you ever seen a naked chick riding a clam before?" (1F07)

Homer is on the roof adjusting the TV antenna. At one point, Bart says "Ooh, psychedelic." (MG48)

Marge's patch on the family quilt is a yin-yang with the words "Keep on truckin'" ("I didn't understand it then, and I don't understand it now") (7F14)

The secret ingredient in the "Flaming Homer" drink is Krusty Brand Non-Narkotic Cough Syrup. (8F08)

In the episode where Bart gets a girlfriend, Skinner confesses "I smelled some marijuana smoke in Vietnam" (There's context behind this that I'll skip). (2F04)

In a cheesy '70's movie Homer is watching ("Goodtime Slim, Uncle Doobie, and the Great Frisco Freakout", if you're interested) rather than satisfying his horny wife, Troy McClure has the priceless line "Here's the bag with our diamonds, but where's the bag with our stash?" (2F07)

In the episode where Homer and Ned becomes friends, the two families have a picnic together. Bart offers pixie stix to Rod and Todd, who aren't allowed to have sugar, in an unmistakable spoof of the drug pusher stereotype. Rod and Todd get a sugar buzz and start fighting with each other. (1F14)

Barney smokes a cigar without removing the plastic wrapper and thinks that Sgt. Pepper is growing out of Homer's back. (2F14)

In Treehouse of Horror III, Burn's throws a sleeping gas grenade at Homer the ape, but ends up inhaling it himself. "I was strolling through the Gas one day..." (9F04)

In the cliffhanger where Burns is shot, Chief Wiggum, upon running out of coffee at the police station, drinks some warm cream, falls asleep, and has a very strange dream about the murder. Wakes up, and Eddie has thought of exactly the same thing. Wiggum: "Did you have that same backwards-talking dream with flaming cards?" Eddie: [pause] "I'll drive." (2F20)

When Bart gets his appendix out, Dr. Nick is the anaesthesiologist and inhales some of his own gas. "Whoopsie. Maybe if I fiddle with these knobs. Hey, I smell gas...pleasant gas. [sighs] Night-night gas... [collapses]" (2F32)

In the X-files episode, Burn's anti-aging drugs leave him disoriented and happy: "I bring you looooove" (3G03)

In the episode where Bart trades Santa's Little Helper for "Laddie", the Lassie-like wonder dog, Santa's Little Helper new owner is a blind man. At the end of the episode, Bart breaks in to get SLH back, the police show up, and Bart arranges to trade dogs. Laddie goes to his new owner, sniffs his pocket, and noses out a bag of weed. Wiggum is about to arrest him when Eddie points out it could be for medicinal purposes. The episode ends with two more cops showing up with a hooker and Bob Marley wafting out from inside the house ("Jammin'"). Wiggum, obviously stoned, sings along. (4F16)

At Homer and Marge's high school prom, a student shows up with a bong. The chaperone lets him in when he is told it is an asthma inhaler. (7F12)

The episode where Homer discovers his mom was a hippie is full of drug references, primarily that fact that Homer mistakenly harvests a bunch of peyote and adds it to the juice, after he ruins the first batch. Shots of various townspeople tripping out culminate with Chief Wiggum tasting it and pronouncing "Why, that's nothing but carrot juice and peyote!" (Fall 1998)

In the Apu Valentines Day episode, Apu says he's surprised to learn that other convenience stores aren't open 24 hours, since 11:30 is the peak time for stoned teenagers. The scene switches to a stoned Jimbo entranced by a piece of aluminum foil: "It's like a living mirror" (Feb 99)

In the episode where the Simpsons have a cell phone tower installed on their house, Homer and Lisa stop by Karma-ceuticals (a store selling alternative health products which has a double hammer banner from the Wall hanging from the ceiling) and spend some time in sensory deprivation tanks. Lisa has the priceless line "How am I supposed to hallucinate with all these swirling colors distracting me?" Meanwhile, Homer hallucinates that Repo Depot comes to repossess everything in the store and hauls his tank away (with him in it). The tank falls off the truck, and homer gets banged around a lot. When he emerges from the tank he's bruised and disheveled. (Feb 28, 99)

In the episode where Mr. Burns loses his fortune and makes it back by starting a recycling center, his first step toward regaining his fortune, at Lisa's advice, is turning in cans to a recycling center run by an old hippie. Burns: "My first dollar! Thanks to you, Lisa. And our hemp-smoking friend: Shine on, you crazy diamond!" (4F17)

In the episode where Homer Changes his name to Max Power, he meets Woody Harrelson, whose pants, which we are told are made entirely out of hemp, look like a bunch of pot leaves sewn together. (AABF09)

The military academy episode starts with Bart's class visiting the Police Station, where Wiggum gives them a tour through a "Museum of Crime". Wiggum: "Now, what I am about to show you next may shock and educate you. Hold onto your values as we step through the looking glass into a hippie pot party." He turns on the lights to reveal a mannequin with a joint taped to his mouth. (4F21)

Ned's Beatnik parents smoke pot. (AABF06)

There's an episode where Dr. Nick is operating on Homer (I think. Actually, Dr. Hibbert might be operating, with Nick as the anesthesiologist). Nick holds up a syringe, saying "This will make the operation seem like a wonderful dream." Then he shoots himself up and punches Homer, knocking him out. (9F09?)

In the episode where Homer undergoes triple-bypass surgery, Nick Riviera (who operates on homer) has a flashback to his medical school days. He's at a pretty sleazy party, hitting on a girl by bragging about how when he's a doctor, he'll be able to prescribe any drug he wants to. (9F09)

In the episode that sees Homer sent to the South Pacific as a missionary, while there, he takes to licking toads, producing hugely dilated pupils and hilarious psychoactive effect. Bart via radio: "Dad, have you been licking toads?" "I have not not been licking toads." (Broadcast 2/21/00)

...and that's just the tip of the iceberg. I'm sure there are many more that I've missed.