Pictsies also allegedly carry their babies with them into battle, if only as a backup weapon. Not necessarily a poor choice, as male infant pictsies evidently use their cradle as a boxing ring.
In Carpe Jugulum the Pictises give King Verence a rather large cup of their battle brew. In the following battle to recapture Lancre Castle, one of the weapons mentioned is a "ballistic King", doubling as Grievous Harm with a Body.
For Discworld's own Ur-example, there's Conina the Barbarian Hairdresser. A beautician by preference and a Badass by genetics (being Cohen the Barbarian's illegitimate daughter), she combines her two aptitudes by using curling irons, crimps, and other salon implements in combat. Her Discworld Companion entry speculates that Conina might convert absolutely anything — a hairgrip, a piece of paper, a hamster — into a deadly weapon in a pinch.
Rincewind speculated that the man she stabbed with the scissors was probably better off than the one she raked on the face with the steel comb.
In Wintersmith Tiffany becomes the owner of a Cornucopia, which produces food on command and can also be used as a weapon...
One big pumpkin, her Second Thoughts urged. They get really hard at this time of year. Shoot him now!
Sam Vimes's butler Willikins grew up on the streets of Ankh-Morpork. He used a cap with sharpened pennies fastened on the brim. Since Terry Pratchett is fond of historical trivia, this is actually Truth in Television.
After his house is broken into in Thud! Sam reflects on how comforting it is at times like this to have a butler who can throw a common fish knife so hard that it is quite difficult to remove from the wall.
Dwarf Bread. Implied to contain gravel as a major ingredient, it lasts for centuries, can be thrown with deadly force and accuracy, and forms a major part of dwarf culture. And in a desperate situation, you can even eat it. It would have to be very desperate though, it's use as a food source is primarily that when faced with the prospect of eating Dwarf Bread everything else looks damn tasty by comparison.
Gussie Two-Grins (mentioned briefly in Night Watch) was one of these, as Vimes describes him, anything was a weapon. Of course this was far from the only mention of 'not strictly speaking weapons' in the book, indeed mentions are made specifically of broken-off bottles, meat-hooks and other tools of butchery, and in one case, a hammer, wedges, and ginger, among many others.
In the same book there's a mention made of an apprentice who attempts to use a broken-off bottle as a weapon in the tense stand-off at the Watch House in Treacle Mine Road. Unfortunately, he does it wrong...which results in the rather tightly-gripped bottle entirely shattering into sharper-than-razor fragments. Vimes calls the only doctor in the city worth being treated by, and between the first aid Vimes applied and the doctor's surgery, the man keeps his life and his hand will work again. This goes a long way towards defusing the tense situation.
Vimes himself, of course, has been known to wield one of his wife's pet swamp dragons as a potential flamethrower.
The Assassins' Guild Diary reveals that among the assorted lethal weapons employed for inhumations by famed guild-school graduates, the Guild museum houses a one-armed teddy bear names Mr. Wuggle.
Rincewind, the failure wizzard once challenged a child Sourcerer with God-like powers with a brick in a sock. Later when he and the same child are faced against Eldritch Abominations, he is thankful socks comes in pairs and fills that one with sand to attack them so the boy could escape.
Nanny Ogg's cat Greebo has on occasion served as what basically amounts to a feline landmine of the "Bouncing Betty" variety.
Yamani (read: Japanese) ladies in Protector of the Small use silk fans with blades hidden in the edges for combat and for playing catch. Kel explains to her shocked friends that they're used when a woman thinks she's going into a dangerous situation but can't wear a weapon openly.
"There is a saying in the Islands. Beware the women of the warrior class, for all they touch is both decorative and deadly."
There's a memorable scene in The Return of the Condor Heroes by Louis Cha when two guys duel. One takes out a fan and asks for the the other man's sword or spear to be shown. He responds by taking out a brush. Guess who's better?
Samson kicks ass with an ass's jawbone. (Animal jawbones with flaked flint chips wedged into their tooth-sockets were actually used as primitive cutting implements by many Neolithic cultures. Hey, it's easier than carving a saw from scratch out of wood.)
Shamgar, another of the Judges, gets only one verse in the Bible, declaring that he killed 600 Philistines with an oxgoad... a stick used to make oxen move forward. From tent stakes to jars with torches in them to farm implements, one might get the impression that the ancient Hebrews considered it cheating to fight using something that was actually intended to be a weapon. And when you're fighting an oppressor, normal weapons aren't easy to come by.
Manly Wade Wellman wrote a series of horror/fantasy stories and novels, the "Silver John" series, in which a wandering minstrel named John battles evil in the backwoods of Appalachia armed only with a silver-stringed guitar. Silver is said early on to be the one thing Satan fears, and that's who John is going after. (His strings were made from old Spanish coins; his grandfather tried to fight the Devil using strings made from silver dollars, but ended up dead — silver dollars haven't had silver in them for years. "The government of men is in league with him.")
A character in Stephen King's novella The Langoliers uses a toaster wrapped in a tablecloth to do some creative rearrangement of their resident psychotic's skull. Amazingly, said psychotic survives this, proving useful for once when the title entities arrive.
A canon Slayers short story has Lina's father ward off a mob through expert use of a fishing rod. The attackers are initially incredulous, but once he demonstrates the ability to flick the hook precisely into his target's eye, no one is too eager to attack him any more. He also later uses it to land a blow on a dumbstruck demon.
In the novella Omega Rising, Omega's weapon runs out of power and she's forced to improvise. Her choice of weapon: a motivational poster.
The Looking-Glass Wars by Frank Beddor contains Hatter Madigan. His weapons include blades that come out of his wrist pieces, a backpack that looks like a swiss army knife when he wants something to fight with (including corkscrews), and a hat that he can split into blades that work as boomerangs. Since he comes from a place where imagination is one of the most important things to have, his crazy weapons are not impractical. He is well known as a very good fighter.
Nicholai Hel (of the book Shibumi) practices a martial art where anything can be used as a weapon. He has used the usual means of killing, but he has also made use of everyday things such as a key, a plastic cup, an ID card, and a folded magazine.
In Up Periscope by Robb White, an American frogman ambushed and killed a Japanese officer with a bottle of Sake.
In Lamb to the Slaughter by Roald Dahl a woman kills her husband with a frozen sheep leg. No, she's not a Lethal Chef, she crushes his skull with it. Then she roasts the sheep leg and feeds it to the police.
In the first Florida Roadkill book, a man is brutally beaten and eventually killed with a variety of convenience store supplies for insulting the cashier. The killers then pay for the items they used as weapons (and the drinks they had entered the store to purchase) and go on their way.
In the horror novel Stage Fright, the VR artiste's previous works include a "dreamie" in which two ghouls battle each other with shovels in a cemetary. It's unclear if they're human graverobbers or monsters, but either way, it's justified that they'd be good at wielding them.
Percy Jackson chokes the Nemean Lion with space meals - in the middle of the Air and Space Museum. If that's not improbable what is?
In Tom Swift and His Airship, towards the end, there is a fight and Mr. Damon sprays a baddie with selzer and then beats him with the bottles!
... Mr. Damon rushing toward the now disabled leader, playing both bottles of seltzer on him. Then, when all the liquid was gone the eccentric man began to beat Morse over the head and shoulders with the heavy bottles until the scoundrel begged for mercy.
In Titus Alone, an enraged Flay attacks Steerpike by throwing a live cat at his face.
Not exactly "wielded", but the protagonist of Pest Control verbally invokes this trope when he realizes he can use New York City as a "weapon" against the foreign hitmen on his trail. Like, say, leading one gun-brandishing killer through the kitchen of an Italian restaurant, then on into its dining room filled with Mafia dons and their trigger-happy bodyguards.
In Harry Potter and the Philosopher's Stone, the weapon itself is not exactly improbable but the manner in which it is used definitely qualifies. When Harry and Ron rescue Hermione from the troll they have locked into the girls' bathroom with her, rather than using it to cast any sort of useful spell, Harry accidentally shoves his wand up the troll's nose (It Makes Sense in Context). The distraction caused by having "a long bit of wood up its nose" allows Ron to use his own wand in a more conventional manner, using "Wingardium Leviosa" to raise the troll's own club high into the air and knock it out.
The Old Kingdom series: The Abhorsens. Magic bells. Also on a handful of notable occasions magical rings that turn into cat collars with magical bells on them.
In Belisarius Series Antoninia's signature weapon is a meat cleaver which she once used to fend off attacking thugs. Later Valentinian who is a perfectionist with regard to hand to hand combat goes out of his way to insist that Rana Sanga's son learn how to use household accessories as weapons; even a prince might someday find himself without a sword.
In one Serrated Edge story, an ordinary human takes down a Banshee with a bottle of (blessed) soda water (containing iron filings). When asked how he managed to get cold iron into water, he replied "Never piss off an engineer."
Musashi is somewhat at a loss when he fights an opponent using the chain-and-sickle as he had never seen it in combat before.
The Spartans of Mythos Academy are born knowing how to turn any item they can pick up into a weapon.
Trainspotting: Bar brawler Francis Begbie likes to carry around a variety of strange weapons, including sharpened knitting needles. Renton suspects that without his weapons, Begbie really isn't all that tough.
In the Rainbow Magic series, in Isabella the Air Fairy's book, a goblin weaponizes cans of air freshener to keep the girls at bay.