Similarly, Ozymandias. Ashtray. Fork. Bowl Cover. Place Mat. Less Than Lethal Results. Most of which probably wouldn't work in real life. But then again, it is Ozymandias.
Likewise, The Question. Potted Plant. Computer Monitor. Car.
Bullseye can use anything as a weapon, usually by throwing it. One example where he is in prison, with his hands restrained. He is using a straw to drink his fresh-squeezed orange juice. Occasionally, he sucks out a pit, and fires it through the straw. To kill flies. But when the ninja show up to kill him, he is all out of pits.
During another prison stint, he was not restrained, but he was denied access to any hard object. So he created his own ammo: he bashed his face into the concrete floor and spat one of his broken teeth in a guard's eye.
At another point when he was beaten to paralysis (it happens to him a lot), he had to be kept on liquid food so that he couldn't throw his stool as a deadly weapon... which Bullseye admits would probably work, and he likely would do so if he could.
To take on Venom in Sinister Spider-Man, he lobbed a yap dog, which ended up clamping onto Venom's eye for the entire fight! Even more amazingly, the dog lived through the ordeal.
On an odd occasion, Daredevil rushed at Bullseye in the corner of a room far away from anything he could actually use as a weapon, figuring he'd be an easy target. Bullseye actually wound up winning and nearly killing Daredevil by beating him against the walls and floor.
Spider-Man's enemy Boomerang, being a former baseball pitcher, can do the same thing as Bullseye (he's just not nearly as good as it) and he displayed this ability during the Deadly Foes of Spider-Man limited series. However, true to his name, he prefers to use boomerangs.
Hawkeye from Ultimate Marvel throws his fingernails through the skulls of several guards in an act of desperation. He's later seen firing pieces of rebar out of his bow.
This is canon for his 616 counterpart as well. He's been known to effectively use everything from pocket change to playing cards when he doesn't have a bow, and has stated that he looks for potential throwing weapons whenever he enters a room.
In the first volume of Resident Alien, Carter Blaine was murdered with a knitting needle.
In an issue of The Avengers, the team encounters a future version of Franklin Richards. He proves he's who he says he is by relating a childhood anecdote where Black Widow showed him how to make poison by mixing his crayons with bathroom soap, and then explained how to strangle someone by using his jump rope.
Spider-Man has been known to use his webbing for offensive situations, such as forming a bat like object out of it, or more famously, Ben Reilly's "impact webbing".
Spider-Man's used quick drying cement to defeat Hydroman;
Fire hose on Electro;
An industrial-size vacuum cleaner to vacuum up the Sandman;
Cathedral bell on Venom;
Whipped up acid to melt Rhino's suit and Doc Ock's arms;
Threw a doom bot at Dr. Doom;
Another, time he used a semi truck on the Juggernaut (didn't work).
Absorbing Man once got Spider-Man to back off with the threat of tossing a commercial airliner.
Rhino once bashed the Silver Surfer over the head with a gum ball machine.
In their first encounter, The Punisher managed to defeat the Russian (an ungodly powerhouse who shrugged off a knife to the gut like it was nothing and casually tore apart a pistol with his bare hands) using only a hot pizza and an extremely fat man.
Garth Ennis's "Welcome Back, Frank" storyline is full of instances of the Punisher using improvised weapons with lethal results. The best example is when Frank gets trapped by mobsters in the Bronx Zoo without ammo and uses the animals to kill them off. He throws one guy at a snake, dunks another one head first in a piranha tank and leads the rest into a polar bear pit. Unfortunately, the bears are more docile and curious than Frank expects, so he punchesapolar bearin the face◊ to get it angry.
During Mike Baron's run, the Punisher was undercover at a South American resort. He spots a hood at the next table who might blow his cover. So the Punisher orders lunch; a club sandwich, a mai-tai, and a side of their hottest salsa. The sandwich came with toothpicks with tufts, and the drink came with a large straw. He dips the toothpick in the hot salsa, and loads it into the straw, making a improvised blow gun. On his way out, he blows the toothpick/dart into the hood's ear canal. The hood goes into fits, with hot-sauce leaking out his wounded eardrum. Frank just goes on his way, like a boss.
Gambit's power is custom made for this trope. Aside from his standard playing cards, he's charged a wad of gum, his credit cards, an anchor, poker chips, a bowling ball (stolen from a pair of guys who look suspiciously like Walter and Donnie), and in one particularly memorable scene, a boysenberry pie.
Does it really count as an improvised weapon, though, if you get to charge it with explosive psychic destruction before you throw it?
Death's Head from Marvel Comics is willing to improvise weapons from whatever is at hand, including furniture, barbecue skewers, and doors. If the room isn't empty, he's armed.
Marv from Sin City has made great use of his surroundings in order to bash enemies' brains in.
Tintin frequently makes use of this trope to get out of the unusual situations he's thrown into; for example, he knocks a butler out using a phone in Secret of the Unicorn.