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In Artemis Fowl, Captain Holly Short uses a cot to beat her way through concrete so she can reach bare earth and plant an acorn in it to regain her powers.
In the second book of the Old Kingdom Trilogy, Lirael, Sam and his classmates fend off zombies with cricket equipment. (Note: the book pre-dates Shaun of the Dead by three years.)
In the Stephen King novella The Langoliers, Albert (teenage violin player) not only uses his cased violin as a weapon at one point, but improvises an even more destructive one from a tablecloth and toaster.
Carried over in the screen adaptation. Albert swings a mean toaster.
Also, at least in the novel, coins/keys between the fingers were pondered as a weapon. The character in question is basically Michael Westen, though.
Another King example is The Mist, where apart from one handgun, the protagonists have to scrounge monster-fighting weapons from the supermarket in which they are trapped. One elderly woman has great success using cans of Raid.
Lisbeth Salander uses a nail gun and a golf club to save herself or other people, at different points in time.
The Bourne Series, with Bourne using kitchen knives, pens, magazines, hand towels and goodness knows what else, to lethal effect.
Vimes was taught by the best improvised-weapon user he'd ever encountered in a bar fight, a heavily scarred man named Gussy Two Grins, who could even see the weapons in a piece of cloth or a piece of fruit (as mentioned in Night Watch).
Also in Night Watch, where Vimes kills a man with a ruler. A ruler! In a segment on technology for prisons, a case of cell-made shanks were shown; one was a wooden ruler that had been perfectly sharpened from the six-inch point on. So this isn't quit so unreasonable. The area the rebels led by Vimes are 'holed up' in includes the city's main butchery shops. Falls somewhere between Kitchen and Farmland, and maybe edging towards Nightmare (a sword is the sort of weapon you expect to deal with, a meat-hook will probably catch you off-guard), especially when they're used by men with as much or more experience with them (20 odd hours a week or more) than most soldiers have with their swords.
Goes badly off the rails for one would-be bottle fighter. He's never done it before, and screws up breaking the bottom out of a beer bottle, causing the whole thing to shatter into sharper-than-razors fragments. While held firmly.
In Snuff, we see Vimes' instinctive consideration of this trope as he visits a cadre of townsfolk with their work tools and offhandedly notes how much damage each of them can do if used (im)properly. In the climax, he's without his sword and has to resort to one such, a steel toolbox.
And then there's Conina the Barbarian Hairdresser from Sourcery. "The one she stabbed with the scissors was probably better off than the one she raked with the (steel-toothed) comb."
There's also Tiffany Aching, who takes out elves with a frying pan.
Mariel, the spunky anthropomorphic mouse girl of the Redwall series. She was thrown into the sea, tied to a piece of wood. The wood floated to shore, and she was attacked by a seabird, which she proceeded to beat away using nothing but the knotted end of the rope she'd been tied with. She took the rope as her favoured weapon, calling it the "Gullwhacker".
Jonathan Hemlock, of the Eiger Sanction and the Loo Sanction, is the master of this. None of the martial arts instructors can figure out how he keeps passing his requirements, until they try to teach him a lesson.
Slippery Jim DiGriz, in a Stainless Steel Rat Goes To Hell. Not to spoil too much, but there's a need to go to an area that doesn't let electronics or machinery work. So Jim arms a company of Marines with three-foot-long salamis. After they are used to smack down the bad guys, a secondary, peaceful application are found.
Plus there's the time he's on the Big Brother planet and can't smuggle in weapons. He improvises a sap by stuffing coins in a sock while hidden from prying eyes under his bedsheets.
In William Boyd's spy novel Restless, the main character is trained as a spy but her handler tells her she doesn't need actual combat training, and that her survival instincts will serve her well enough if she's in a dangerous situation. Later in the book she kills an armed assassin by stabbing him in the eye with a pencil
World War Z has the Lobotomizer, essentially a bladed shovel made of old car steel. At this point, the government is so badly reeling that it accepts the "Lobo", and it becomes a signature anti-zombie weapon.
Truth in Television: During WWI, those fighting in the trenches would sometimes sharpen their entrenchment tools (AKA small shovel), since they were often better weapons than the bayonets.
One soldier mentions the time he once saw The Big Guy of his squad use a zombie as a club.
In Neal Stephenson's Reamde, Zula Forthrast uses the broken shards of a DVD of the film Love Actually to kill a terrorist.
In Mercedes Lackey's Winds of Fate, Kerowyn makes a point of training Princess Elspeth to think of anything around her as a weapon, which promptly proves its value when an assassin attacks her and she breaks a vase and cuts his throat with it.
Later it becomes a bit of a joke between the two of them.
The second Gears of War tie-in novel, "Jacinto's Remnant" features a battle in a general store between COG and Locust forces in a flashback to the year 1 AE. After Marcus's bayonet breaks trying to pierce a Drone's tough hide, Tai saves him with a nearby circular power saw. This served as the in-universe inspiration for the series's signature Chainsaw Bayonet.
In Area 7, broken pieces of airplane are used as swords and shields.
Innocent items put to lethal use are one of humanity's distinguishing traits in the Known Space series. It particularly makes a point of how any high-tech equipment that involves a laser can be modified into a lethal weapon (including a highly advanced adjustable flashlight), but it also has a lot of smackdowns with mining equipment.
Another notable offensive (and defensive) use of non-combat technology involves a group of humans (in pressure suits) taking on a group of 8-foot tall Kzinti warriors. The only one they're worried about is the one in the armor, which is handily taken care of with a power drill to the chest. The rest of the Kzinti are practically helpless, as their instinctive mode of attack is to slash with their claws. These humans are wearing industrial pressure suits, the kind used by asteroid miners and other people who need suits that are highly resistant to being cut or torn.
A professional hitman in Safe Harbor by Eugene Izzi has taken this trope to heart, since infiltration's a lot easier when you're not obviously armed. His Weapon of Choice is a credit card that he meticulously sharpened one edge of, and he can actually slit people's throats with it.
Played with in Pest Control, when the hitman-hounded New York protagonist realizes that his best weapon to fight back is New York City, itself. Say, by slipping through a certain restaurant's back door with a killer hot on his heels, luring the gunman through the kitchen, and then ducking for cover as both emerge into a dining area packed with mafiosi and their trigger-happy bodyguards.
In Dean Koontz' "Odd Thomas" series, the titular main character states that he hates using guns, but can, will, and has used anything else on hand as a weapon.
In Blackout, Cal Leandros, who is normally happiest using large-caliber firearms, kills a spider the size of a German shepherd — with a fork.
In The Honor of the Queen, Honor uses a metal tray as a thrown weapon against Protector Benjamin's would-be assassins.
Time Scouts prefer to stay invisible. Failing that, they prefer to use their favorite weapons. Failing that, they'll use whatever they can lay their hands on.
In an explicit shout-out to Buffy the Vampire Slayer, Harry Dresden tries to turn his recently-broken blasting rod into two stakes for fighting vampires. It doesn't work. He's also been known to hold mouthfuls of garlic and let vampires tackle him, and use a (steel) letter opener and nails to ward off Fae. He's killed a demon with an elevator. He was much more fond of this earlier in the series.
Toot Toot saved Harry from the skin-walker when Toot Toot tried to attack the skin-walker with a box-cutter and wearing makeshift armor. Remember, Toot Toot is the size of a G.I. Joe action figure, the skin-walker is (at the moment) about ten feet tall, and is always one of the heavy-weight eldritch abominations in the series.
Thomas Raith broke a chair over another White Court vampire. The chair was made of metal.
A badly constructed Entropy Curse will do this. Bees? Electric shock in a pool of your own blood? A car, while you're jet-skiing?
And, in what may be the single funniest scene in the series, a frozen turkey falling out of the sky.
Harry once confided to the reader on the merits of a ring of keys as a projectile weapon. And tried to trip up a chlorofiend (read: plant monster) with gumballs.
Murphy melted down silver earrings and forged them into bullets.
In The Savannah Reid Mysteries, Atlanta saves her sister Savannah's life by smashing her guitar into the murderer's leg, breaking both the leg and the guitar.
In Manly Wade Wellman's short story O, Ugly Bird John uses his guitar to kill the monster. It's strung with silver, and when he smashes it onto the creature, the silver kills it. John removes the strings and puts them on a new guitar.
In Fate/Zero, Berserker's special ability turns any object in his hands into a super-powerful weapon and also makes him an expert at using it, even completely random objects.
A heavy stone paperweight is pressed into service on two separate occasions in A Brother's Price.
"Ivar's brother Leif beat one of Steinn's men to death with a rib of the whale. Then they fought with anything they could get.”
Roald Dahl's Lamb to the Slaughter involves a woman using a frozen leg of lamb on her philandering policeman husband, to lethal effect. Then she feeds it and the rest of the dinner to his colleagues who come to investigate.
In Doom, Arlene likes to make creative use of things around her. First was her Chainsaw Good moment on Deimos. On Earth, she uses a fire extinguisher to surprising effect on an arch-vile.
At one point in The Legion Of The Damned, a group of human miners are trapped on a small asteroid by a massive armada of alien warships. They utterly shatter the fleet my destroying all their battleships (and thus eliminate their entire command and control capability) by ramming them with the mass-driver launched drones they use to scoop-mine the local star for dust that glitters in a very pretty way.
In the Terry Pratchett/Neil Gaiman novel Good Omens, Crowley grabs a tire iron to fight Beelzebub with. Ironically, this would be better than an actual weapon, because "It wouldn't be any good, but then, nothing would. In fact it'd be much more terrible facing the Adversary with anything like a decent weapon. That way you might have a bit of hope, which would make it worse."