One could use just anything as a weapon. Just think of a random object, you probably can use it to choke/strangle/suffocate/stab/cut/bludgeon/blind/crush/poison/corrode/shock/throw at/enflame someone.
Medieval polearms got their origins from farming implements.
Hans Talhoffer's fighting manual from 15th century Germany deals on fighting with improbable weapons, like farm tools. It also includes the infamous "How to use a club to defend oneself against a woman wielding a shawl wrapped around half a brick when you are buried to the waist in the ground".
A convenience store worker worker was faced with an attempted robbery by someone armed with a banana, according to an Uncle John's Bathroom Reader book. Naturally, things didn't work out well for the robber, who was arrested after failing to steal anything after hitting the clerk with his "weapon."
One convenience store in Florida was the victim of an attempted hold up by a man using his shirt as a disguise and armed only with a palm frond and a terrible quasi-Russian/Arab/Japanese accent. He too was a victim of this trope when the clerk at the time pulled out a deadly wooden stool and shooed him out like a stray cat. Thus was born the legend of the Palm Frond Bandit.
For the purposes of convicting Carl Eugene Watts, a Texas Serial Killer, bath water was ruled a deadly weapon. Specifically, he agreed to plea to the charge of Burglary With The Intent To Kill, wherein the fine print of the law meant that if he used "a deadly weapon", Watts would not be allowed to deduct time for his sentence for good behavior. While he indeed tried to kill the owner of the apartment by drowning her in her bathtub, Watts successfully appealed the ruling, but would spend the rest of his life in jail due to a later conviction for a murder that occurred in Michigan (which did not agree to the plea deal).
The Russian Special Forces have combat techniques that use their trench shovels, because "they are as maniable as knives, but have more range and striking power".
If Erich Maria Remarque is to be believed, a shovel is a better weapon than a bayonet in the trenches. After all, several feet of solid wood with fairly sharp chunk of metal on one end? Reminds me of something, only better for trenches. Plus, y'know, they're easier to dig with.
Though not nearly as effective as his fictional counterparts, sleight-of-hand artist Ricky Jay holds the Guinness World Record for throwing a playing card the furthest, is capable of piercing the rind of a watermelon with a thrown card, and wrote a book titled, "Cards As Weapons".
Professional Wrestler Sid Vicious got his ass kicked by Brian Pillman, stormed out of a bar, and came back armed with a squeegee.
Russell Crowe's incident with a telephone probably counts for this.
In Arab countries, hitting someone with a shoe is an extreme form of insult, since the feet are considered to be very unclean. This became much more widely known during the second Iraqi War, in which a statue of Saddam Hussein was struck with shoes, and a protester even chucked two shoes at George W. Bush.
Hawaiian swords (a wooden paddle with shark teeth along the edges).
Aztec swords, Macuahuitl, were set with blade shards of obsidian, which can get surprisingly sharp. Cortez claimed that the blades could cut through a horse (tested on Mythbusters with mixed results). They also used slings with obsidian bullets that would shatter into sharp shrapnel.
Obsidian, though rather brittle, functions as an Absurdly Sharp Blade. It's crystalline structure makes it considerably sharper than steel, to the point where some surgeons have started using obsidian scalpels.
A "kung fu" long-sleeved robe with lead weights sewn into the sleeves.
Also French police used to wear short capes with weights in the hem and would whorl them around in riots.
Fencers learned to use their capes as a parrying device in the absence of an off-hand weapon.
'Duckfoot' pistols with 5 barrels on one trigger, for people who don't care who they hit. Intended for riot control situations, where you've got a lot of enemies.
A cutlass with a pistol in the handle (yes, a freaking gunblade).
A Soviet combat knife with a single-shot pistol built in the handle. There's also a Chinese version that fires four .22LR rounds.
A man was charged with assault for smearing peanut butter on a doorknob. The victim was allergic, but it still warrants a mention.
In Sweden, a metal snuffbox is colloquially known as a "slagsmålsdosa" (fighting box). And yes, there are many recorded instances of people fighting with them, though they have lost popularity since The '60s.
The famous samurai Musashi is said to have defeated his rival Kojiro with an extremely long wooden sword he quickly fashioned from an oar. Although it sounds bizarre, this is quite logical: reach is a big decider in a swordfight, in most swordfights you're meant to deflect the opponent's blade with the flat of your blade, and getting hit with an oar hurts like hell.
Okinawan villages were regular targets of evil lords who twirled their mustaches while cackling. Some Japanese fishermen, in response, made an entire martial art half-based off of two-handed swordplay, half-based off of staff combat, all using an oar. And won. This obscure but deadly martial art is called "chikin sunakachi", or "ekudi".
Bartitsu and La Canne are two martial arts devoted to defending yourself with your walking stick. When so many gentlemen carry around a wooden club with a weighted metal head, it's only natural that they tend to get used in brawls.
Another important technique taught in Bartitsu was using an overcoat like a net to ensnare your opponent's head and arms.
During World War II spies were taught to make a dagger by repeatedly folding a newspaper into a small square and thrusting the point into the throat of an enemy.
Some prison inmates harden sheets of toilet paper using toothpaste, and file them into shivs.
Molten chocolate can be weaponized, and is used by prison inmates. Melt enough candy bars and you're armed with burning hot goop that sticks like napalm.
Another improvised prison weapon uses a plastic coffee lid note if such gratuities are available in a specific penal system, which is partially heated over a burning roll of toilet paper to become malleable. The inmates then roll and twist them into a point, with the rest of the mass shaping neatly to the wielder's finger grooves. Perfect for jabbing someone in the throat.
Under most historical dueling codes, the challenged party was allowed to choose the weapon. Some people were known to ask for outlandish things like spoons or cannons to illustrate their disdain for the practice without the implied cowardice of declining the challenge. US President Abraham Lincoln was a notable example of this, once declaring that a duel was to be fought with the two fighters separated by a plank so that his opponent couldn't reach him. The challenge was withdrawn.
In a similar story, the Iron Chancellor Otto von Bismarck once challenged German doctor and political rival Rudolf Virchow to a duel over the latter's strenuous opposition to the former's military budgets. Two stories exist as to the outcome of this duel: in the first, Virchow simply ignored the challenge as uncivilized. In the second version, however, Virchow, as the challenged party, selected two sausages as the weapons of choice. One sausage was perfectly ordinary; the other intentionally contaminated with live Trichinella larvae. The "duel" was that each would choose one of the two sausages to eat. As the second story goes, Bismark withdrew his challenge on being informed.
Large, metal framed flashlights as mentioned above are actually extremely common among security officers who aren't permitted to carry weapons on duty.
The meteor hammer is essentially a long rope with weighted knotted ends. In case this wasn't enough, skilled practitioners light them on fire.
There is a company in the Philippines that makes combat umbrellas. They are strong enough to split a watermelon, or support a man's weight, and can still be used as an umbrella.
In areas like Vancouver with frequent rain, umbrella self-defense classes may be offered, and most citizens are able to do so even without training through sheer amount of use.
To wit the Island of Guernsey has seen the following weapons occur:
A bottle of Fanta Orange Soda (Contents only), which ended with an assault charge.
A pig (Thrown)
A copy of shogun assassin, box and all, fired from a modified crossbow.
A flaming rabbit carcass
A 3/4 length men's coat (As a flail, weighted down with pocket change)
A bottle of makeshift napalm, the neck stoppered with a broomhandle. (Used as a pole-mace)
A crate of Stella Artois beer bottles, thrown as a whole.
An angry terrier harnessed onto a broom handle.
Highly compressed snow in the shape of artillery shells, fired from a modified ballista.
During a reenactment-grade sword training lesson, a training stick was broken with a wooden waster, one person retrieved a foot long section of it and used it as a training dagger, successfully disarming and taking down his sparring opponent.
Historically, hunters in the Philippines used a "stone yoyo"— basically, a vine and a disc of rock. They'd sit in a tree, leave a bit of food at the bottom, wait for an animal and PONK!
The Battle of Mactan. The natives threw everything at Magellan's men— including but not limited to shellfish, coconuts, durian fruit and their own shit.
It's worth noting that a ripe durian can be used as a mace. Some people in other parts of Southeast Asia have been arrested and fined for attacking people with this fruit-mace, and people going into durian farms have to wear construction helmets.
A Florida man was sentenced from throwing venomous jellyfish into crowd.
The Japanese have a long and proud tradition of developing elaborate ways to use pretty much anything as a weapon, but the one that takes the cake is the fact that some samurai had pipes (the smoking kind) made with the same weight, length and balance as a sword, for use in places where weapons were prohibited.
During the early days of the AIDS epidemic, at least one HIV-positive suspect was charged with assault with a deadly weapon for spitting at the officers who arrested him, although the charges were dismissed when the fact HIV isn't transmitted by saliva was pointed out.
Some other interesting examples from Japan. The kunai and kama were developed from a masonry trowel and a farmer's sickle, respectively. Its why these weapons are associated with the Ninja, as they frequently disguised themselves as simple workers or farmers. Among the craziest of ninja weapons was probably the happo. Small eggshells filled with blinding powder, intended to be thrown at an opponent in order to leave them vulnerable to attack, or to distract them in an escape.
According to some sources, nunchaku were derived from a flail device used for threshing rice.
Finnish Army conscripts are not issued bayonets or fighting knives as everyone is assumed to already own one and bring it with them to the army. The Finnish puukko knife doubles well as a cooking utensil, everyday tool and fighting knife.
The Japanese tessen, is this. There were several types, ranging from paper fans with iron spokes used to cool oneself, to larger fans that were solid iron (or iron with a wood handle) that were used to signal troops. Samurai carried them for defense in places where they couldn't carry their swords.
Several Kung Fu styles, most notably Hung Gar, teach horse-bench as a weapon. The idea is that if you were caught unarmed in a tea-house or diner, you could use your seat to fight your way out.