The Hitchhiker's Guide to the Galaxy has the Trope Namer, the Pan Galactic Gargle Blaster. It's the best drink in existence. The psychological effect is like having your brains smashed out by a slice of lemon wrapped round a large gold brick. It's very expensive, and bad for your health. The sixth book, And Another Thing..., written after Adams' death, includes a drink made from dragons' souls.
Ford Prefect once wiped the mouth of a bottle of Ol' Janx Spirit off with his towel before giving it to a girl. This had "the opposite effect of what was intended, in that the fumes of the Ol' Janx Spirit killed off all the bacteria on the towel..." There is also a drinking game based around it, in which two players place a bottle in between them and take turns telekinetically tipping the bottle so it fills up the opponent's cup - which gets somewhat harder to do when you're off your face from the first sip, so it's something of a slippery slope. Ford, we're told, plays to lose.
"Scumble" is a particularly potent form of hard cider (it's frequently described as being "made from apples... well, mainly apples" and was said in the GURPS Discworld to have "some qualities of fresh apples in autumn and some of dimethyl hydrazine before liftoff"). Typically sold in tiny thimbles because overindulging can cause all sorts of interesting side effects, including seeing horrible green hairy things bursting through the walls. See the Real Life section for the drink scumble is based on. The following quote details some of the mythology which is typical of a good Gargle Blaster:
As everyone knows, there's no danger of encountering watered-down scumble — because scumble reacts explosively when it comes into contact with water. Nanny Ogg's specific variant is sometimes known as "Suicider".
There's one humorous scene in Mort where, due to his naivety and duties as substitute Death, the title character orders a pint of the stuff (to considerable surprise), downs the whole thing without being affected (to even more surprise), and then walks straight through the door without opening or damaging it (leaving everyone positively stunned). Rendered very amusingly in the "Big Comic" edition where the view cuts to the horrified customers exclaiming "A pint?!?" in hushed tones when Mort places his order.
Also, during said scene, Mort screams that there's something coming through the walls, but no-one else can see it. This allows the pub patrons to relax, since things going through walls is a perfectly normal Scumble-induced hallucinations, and again when Mort starts yelling that everything looked different a moment ago.
In Monstrous Regiment, the troll equivalent of a Gargle Blaster, the Electrick Floorbanger, is prepared by dropping silver and copper coins into vinegar; the resultant crude battery temporarily shorts out the troll's silicon brain.
Also mentioned in Monstrous Regiment is the rotgut brewed by and flogged to Borogravian soldiers, known as "hangman" - one drop and you're dead.
Trolls also have a drink called luglarr, or "Big Hammer", a variant of troll beer made by adding certain metallic salts to the drink that manages to make it even more dangerous — very hard to do. The result is roughly the same as scumble, to the effect that anyone who can't simply be pushed over minutes after drinking some is considered almost preternaturally resistant to its effects, even by other trolls. To sum up: this stuff etches pavement (and remember, trolls are pretty much made of rock).
Thud! also mentions fluff, a cocktail made by mixing dwarf beer and scumble (which is the non-troll inebriation equivalent of adding gasoline to a fire).
From Sourcery comes "Desert Orakh", which is a mixture of scorpion venom and cactus sap that's been left to ferment in the sun for several weeks. It's actually noted that it isn't drunk as an alcohol, but as a counter to Klatchian Coffee.
In Hogfather, Bibulous the God of Alcohol is about to drink a lovingly-described, heavily-garnished, layered cocktail when he gets hit by the side-effects of the Hideous Hangover Cure consumed by Bilious, the Oh God of Hangovers ("Does anyone hear a slide whistle descending...?").
An early Discworld book mentions the mountain farmers making a strong real-life liquor called applejack by putting buckets of cider out in the winter and letting everything but the alcohol freeze.
In the Tiffany Aching series, there is the often-referred-to "special sheep liniment", a type of moonshine whisky which all the sheep-farmers keep around for cold nights; it is said to put hair on your chest. It is often speculated by people who are unfamiliar with it what would happen if one were to actually give it to a sheep, with the implication usually coming back that they are not sure, but it probably wouldn't be good.
The Last Continent features a mild example of this in a beer known as "Funnelweb" (a type of venomous spider). Except that's not it's name, that's the list of ingredients. It actually manages to turn Rincewindinto an optimist.
And then there's splot, an Überwaldean tonic mentioned in Making Money which might have begun as schnapps. It is made from herbs and other all-natural ingredients, which as everybody knows are good for you. Of course, belladonna and mandragora are herbs and arsenic is natural. Also, splot doesn't actually have any alcohol in it... because alcohol can't survive. Nevertheless, splot is a potent and effective pick-me-up with the unfortunate side effect of speeding the brain up so fast the rest of the body (such as the tongue and limbs) has a hard time catching up.
The Heaving Bosom nightclub for Vegetarian Vampires, mentioned in The Ankh-Morpork City Guide serves a spirit that's similar to absinthe, except it's red instead of green. It's not recommended for people who aren't dead already.
Maple mead from Lois McMaster Bujold's Vorkosigan series is always discussed with trepidation by the main characters. The Dendarii mountain folk, who are backwards even for a backwards planet like Barrayar, don't mess around with their alcohol. In moderation, the drink doesn't have much kick. The first glass or two taste sickly sweet, the next few glasses taste pretty good... and then you wake up the next morning with a killer hangover.
The Yahtzee novel Fog Juice is named for the Gargle Blaster prepared by the protagonist to solve problems. It is said to be a recipe passed down through generations of university students, which can be summed up as every bottle in the kitchen plus a large mixing bowl. Its main advantage is that when you come round, whatever unfortunate situation you were in will definitely have resolved itself — however, you may have a few new problems, the least of which is working out where you are and how you got there. It also renders you completely immune to all other forms of alcohol in the future, even notoriously evil Pirate Grog. And allows you to access the collective human unconsciousness with an avatar of your inner self. For the main character, this is a terrifying floating mass of tentacles which are themselves made of vomit. The female lead, in disgust, wonders pointedly what this says about him.
The most popular drink in the title country of the Welkin Weasels series is honey dew, "known to make angels out of hawks and devils out of hickory sticks." (Translation: It's really strong, though it may just be strong in proportion to the weasels, who are very small and presumably can't handle huge amounts of alcohol.)
The Commissar by Sven Hassel. While behind enemy lines in Soviet uniform, the German protagonists are stopped by suspicious NKVD men. Tiny invites them to take a swig from his bottle. The NKVD men do so, turn pale and collapse. Tiny then drinks from the same bottle with no ill effects.
In one of the Captain Future books, a character orders some drink which the others describe as something like "one ounce makes you feel like being hit with a meteor, two make you become one". Being an android, the character drinks the entire (ceramic) bottle with no visible effect, and then orders wine with radium chloride — which does make him drunk.
The title hero of the Sten series drinks an alien brew called stregg, a vile libation named after said aliens' (extinct) ancestral enemy. Since even the Eternal Emperor — a whiskey-swilling immortal gourmand who is centuries old and brews his own 180-proof moonshine in a car radiator — is shocked by the potency of stregg, one can assume that it is literally the worst rot-gut in the universe.
In the Night Huntress books, Bones gives Cat a bottle of his special moonshine to drink to persuade a recalcitrant ghost. (It Makes Sense in Context). She ends up drinking the entire bottle when she was only supposed to have a few sips. Bones is amazed that she's alive, let alone that she can walk (barely).
"You let her drink a Gravedigger?" The Blood in Anne Bishop's Black Jewels universe have very high metabolisms, especially when their power is deep. This makes it difficult for some characters to get drunk without making use of a Gargle Blaster concoction. Or two. Or seven. On the other hand, if a more mid-powered Blood gets any in them, Hilarity Ensues.
The fourth book of Stephen King's The Dark Tower series, Wizard and Glass, mentions one of these. In the bar in Mejis where young Roland and his compatriots are staying, the bartender dumps the unfinished drinks of all her customers into a keg labeled "Camel Piss", and charges a small sum to anyone who is stupid, brave or desperate for alcohol enough to want to drink it.
Dragon's Blood in You Fire Me Up by Katie MacAlister.
One of the authors of Cooking in a Bedsitter seems to be familiar with them writing that when entertaining "It may seem fun to serve a big bowl of hell-brew full of odds and ends of orange, apple and cucumber.", but advising against it on the grounds that the guests end up being "...sick on the carpet."
In The Gnole this is a pint of Old Pekuliar. The scene goes much as described, as well as people preparing to tell their grandchildren about it.
In the first of Steven L. Kent's Wayson Harris novels "The Clone Republic" the protagonist imbibes a liquor called Sagittarian Crash, a vodka made from slightly toxic (possibly radioactive) potatoes grown on a specific planet. It also immediately blitzes his friend when he unknowingly drank it in a Hawaiian cocktail.