A brief scene in Walt Disney's Peter Pan shows Hook choosing a gold-plated hook from an assortment of hooks.
He also uses several different ones in... the movie Hook.
Phantom, the Big Bad from MÄR, has all of his most powerful Ã„RMsgrafted onto his left arm, which is wrapped up with cloth straps to keep it hidden. It's not exactly surprising when he reveals it, but it is awesome.
Ed has an automail arm, not too rare in his universe, but with alchemy he can pretty much do anything with it. Why does he even need a mechanic? Automail is very complex machinery, and he doesn't have the knowhow or skill to perfectly put it back together after transformations. He goes to Winry so she can set it right again every once in a while. And he does periodically need it lengthened.
A better example would be Paninya, who shows up early in the manga. She's got a blade in one leg and a gun in the other. Both of which prove to be fairly useful against the Elric brothers.
Buccaneer at Fort Briggs is short one arm. Being from Fort Briggs, the prime center of badassery in Amestris, he naturally has several automail weapon/arms, including one which is more or less a pair of chainsaws attached to his shoulder.
And Lan Fan loses her arm and has it replaced by automail, but because she's a ninja, it has a retractable blade.
Guts in Berserk loses an arm to a demon during the Eclipse, but it's all good because his replacement includes a gunpowder Arm Cannon and a repeating crossbow! On the other hand, the thing does not have moving fingers, but powerful magnets make sure that the artificial fingers are tightly grasping his massive sword, The Dragonslayer.
Variation: Shinkuro from the anime Kure Nai didn't lose a limb, but as a child he had a sword-like blade placed inside his arm while he was training with a family of ex-assassins after the death of his own family. It pops out through his elbow when he gets overly agitated in combat, and in the final episode he proves to be frighteningly efficient with it. He now regrets having it put into him, though, and is trying to be strong enough not to ever need it.
Gray from Gunsmith Cats got his hand blown off the first time he ran into the heroine Rally Vincent. When next we see him he uses a hook when he want to be relatively inconspicuous, but swaps in out for a two-foot long spring-loaded steel machete he can launch at will with enough force to punch through a car door.
Don't forget Bonnie from the first volume, who conceals a strangling cord inside her prosthetic thumb, and a shotgun in her leg. And the foot of that leg apparently contains a remote-controlled bomb.
Justimon, Cyberdramon's Mega form, from Digimon Tamers has the Trinity Arm, which has three different forms.
Gundam GP 03 Dendrobium Stamen from Gundam 0083 has arms which open up into long, thin grasper arms. These are not used for hand-to-hand combat, but rather to grab various armaments out of the gigantic Dendrobium Orchis mobile armor that it rides around in.
Swiss army limbs containing holdout weapons are common for the robot bodyguards and cyborg assassins in the various incarnations of Ghost in the Shell, not so common amongst the heroic characters.
Catwoman once fought a foe called Wooden Nickel whose 'power' (and I use the term incredibly loosely) was that he possessed a wooden arm with multiple functions.
The original Death's Head has several different weapons that he can swap his right hand with, including a mace, an axe, a blaster, and several different types of missiles.
Death's Head II has a liquid metal right forearm with (at least) four different configurations: hand, cannon, blade weapon, and "siphon" (which is used to absorb information, knowledge, and skills from others, organics or machines alike).
Cable from Marvel Comics. Early in his continuity his metal arm was one of these instead of whatever the heck it is now.
Aaron Stack, AKA The Machine Man, of Marvel Comics; originally a serious superhero, currently a member of adrenaline-soaked parody team Next Wave. All of his limbs are this—heck, he's like a more competent Inspector Gadget crossed with Bender from Futurama.
"I am full of useful devices."
Lucien Machete from the Topps Comics Zorro series had a variety of weapons and gadgets built into his Hook Hand.
Films — Animated
Japeth the mountain goat from the film Hoodwinked has an incredible number of detatchable antlers that can be used for every practical purpose: including holding his car keys (which is strange since he doesn't own a car), a set can be used as a bottle opener, a set used like a coat rack, and even a set with helicopter blades that allow him to fly.
Gobber from How to Train Your Dragon replaces his missing hand with whatever the situation calls for. A fork, a stein, a hammer, etc. He is even introduced as something along the lines of 'the loudmouth with the interchangeable limbs'. Hiccup gets his own prosthesis at the end, which includes suspension and is neatly designed to fit in to Toothless' gear system.
John Silver in Disney's Treasure Planet is a cyborg, with artificial arm, leg, and eye. The arm transforms into about a million different things, from tools for cooking to a cutlass. He can even use another part attached to his leg to turn it into something akin to a plasma cannon.
In the final segment of the Heavy Metal film, the villain has a mechanical hand. For the final fight, he swaps the hand for a buzzsaw-type cutting tool.
Films — Live-Action
Mr. Han, the Big Bad in the Bruce Lee movie Enter the Dragon, had a hand-stump, to which he attached a jade fist, tiger claws and a bagh nakh.
The Machine Girl (Kataude Mashin Garu) features a Japanese schoolgirl who loses her arm to the mob and replaces it with a machine gun as well as a chainsaw.
Judge Doom from Who Framed Roger Rabbit has a golden one with an anvil at the end of it, which then becomes a buzzsaw.
The film Life Pod had the 'Toolies' - a sub-species of dwarf-sized human's who underwent surgery to have tool adaptors on one of their arms.
In Robert Rodriguez' Planet Terror, Rose McGowan's character, a go-go dancer named Cherry Darling, loses a leg early in the movie and has it replaced by a peg, and later, a machine gun/grenade launcher. That's right, it's a gun leg. Rule of Cool keeps it from exploding from all the dirt that would get packed into the barrel.
Robocop fits this trope more and more as the film series progresses. In RoboCop 3, he gets an Arm Cannon with an underslung grenade launcher, and a data interface on his normal hand. However, one of his forearms has to be detached with his other hand in order to exchange it for the Arm Cannon.
In Evil Dead 2, Ash amputates his own demon-possessed hand and replaces it with a chainsaw to battle the undead. In Army Of Darkness, he replaces that with a mechanical gauntlet strong enough to crush a metal goblet like it was play-doh. Groovy, indeed!
The T-X from Terminator 3: Rise of the Machines has one of these: the liquid metal lets it be a normal hand, else it can fold about into a variety of weapons. The ones we see used are a plasma cannon, a nanomachine injector, and a flamethrower, but her navigational screen indicates there are around two dozen alternates. Including an M-41 Pulse Rifle, apparently.
Innerspace's Mr. Igoe has several devices he can use to replace his missing hand, including a hand which fires bullets from the index finger, a flamethrower, and a vibrator. Amusingly, the license plate of his car is "SNAPON".
In Hook, Captain Hook has a number of swappable attachments for his missing hand, including a cup-like device he uses to hold a glove for the impromptu game of baseball he sets up to try and win the love of Peter Pan's son.
And, unlike most fictional Swiss Army Appendage users, the book is not shy about the problems it causes from straps going loose in battle to the time it takes to switch arms to the additional strain it puts on his body to have to support the unnatural stresses.
Eugenides in Megan Whalen Turner's Queen's Thief series has a few different prosthetics he uses to replace his missing hand, including a cosmetic false hand and a hook that he's used to kill a would-be assassin.
Not an appendage, strictly speaking, but Arno Blunt in Artemis Fowl: The Eternity Code acquires a wide variety of bizarre false teeth at some point after having his real teeth knocked out by a sonic grenade.
Edward from Prizzi's Glory has a collection of dentures modeled on the teeth of famous individuals, including Adolf Hitler, which he uses to reshape his lower face.
Gil Terry from Space Vulture has an alien cricket arm and a telescoping beetle eye. He lost his real body parts in a sports bet and had them replaced with non-species-specific spares, much to his displeasure.
Herzer Herrick, from John Ringo's Council Wars series, uses a nifty prosthetic hand that combines a clamp and a shear with a torque amplification system befitting both the series's Anachronism Stew and his own Canon Sue status. It even opens beer bottles, though he'd much prefer to have a real hand. As both a high ranking officer, and the consort of one of the most powerful women on the planet, he could easily get his hand regrown. It's a matter of personal honor that he won't do so until after the war is over.
Quantum Gravity: When Lila gets these, it is explicitly noted as odd. On the other hand, she's magic-stained and has elementals floating around in there, so what do you expect?
Riderman, from Kamen Rider V3, has this for a right arm. Called the "Attachment" or "Cassette" Arm, it can switch between Normal, Rope (Hook, Mace, and Scythe options), Net, Power (claw), Drill, Repair (mostly for other cyborgs), and Machine Gun(!) Arms - simply by switching out a cartridge in the elbow. This is his only cybernetic adjustment in the original series, which makes him something of a Badass Normal.
The Zect Troopers in Kamen Rider Kabuto use a variation/homage of this, designed to fit over a normal arm and perfectly capable of taking down weaker foes.
Given that Riderman's is the first live-action use of this trope in Japan, and in the early-to-mid-70s besides, it can be reasonably assumed that this is the source of most other Japanese versions nowadays (Barrett Wallace's "Atomic Scissors" resemble the Power Arm, and he's also got a "Drill Arm", for example).
Why would an Obliterator need a crowbar in 40k? The most obvious need would be to open doors or crates, but then we're talking ten-foot mountains of flesh, metal and gun. Chaos isn't famed for common sense either; the whole faction seems to be either The Starscream or the kind to shoot out a door before asking if someone's on the other side.
By making the best of the powerful curse she's under, Jacqueline Montarri has become one of the most adaptable villains in the Ravenloft setting. Having lost her own head centuries ago, she decapitates other women and wears theirs, instead. And she gains any class-based abilities which those unfortunate ladies might've possessed in the bargain. If she ever offers to show you her "collection", don't accept, because it's not stamps or butterflies.
The Infernal Exalted have access to a number of demonic "enhancements," one of which is called the Fourfold Demon Arm. It can become a normal human hand, a claw, a tentacle, or any non-ranged weapon, though each one only has four possible transformations. Still not as nasty of some of the other Body Horror in Exalted, especially if you get into Necrotech and Necrosurgery.
An even better example occurs with the Alchemicals, who are effectively cyborgs, and have an enhancement called Omni-Tool Implant which replaces a character's hand with a device which looks like a hand, but can unfold to produce so many tools that the character is considered to have a master level workshop no matter where they go, which essentially means they have every tool necessary to produce any kind of mundane object, up to and including things that would require a forge to produce. One of the upgrade submodules turns it into a highly advanced piece of surgical equipment.
In Cybergeneration, the Tinman character types have all four limbs replaced with these. A skilled Tinman can do truly impressive things with them, even, if the GM permits it, glide by turning their limbs into airfoils.
In the D20 Modern campaign setting, Otherverse America, one character class, the Midwives, are doctors who can transform their bionic arms into a variety of medical and surgical equipment.
Ideal produced a figure in the 70s called J.J. Armes, a cyborg detective whose main selling point was an arsenal of snap-on hands.
Barrett in Final Fantasy VII replaces his amputated right arm with an Arm Cannon. There are also several weapons for him that aren't guns, but scissors or blades. And a boxing glove. In Advent Children, after he has become a rich oil magnate, he gets an arm that can be changed from normal prosthetic hand to machinegun to laser cannon, etc., at will, without the necessity of detaching it.
Fritz from Brain Dead 13 replaces his hook hands with all kinds of melee weapons, guns, and other props (blender, electric razor, etc).
Mega Man'sArm Cannon was this, in his original incarnation of Rock. This was before he had to be weaponized. May be a borderline case for this trope, as he shapeshifts/replicates things instead of hot-swapping.
Technically, his "Swiss Army Appendage" got weaponized as well into his ability to adapt new weapons.
In Mega Man Legends, Volnutt's right arm is a very literal interpretation of this. While he has his Arm Cannon on his left arm, he also has a veritable assortment of special weapons that he can equip and swap out onto his right arm, ranging from drills, to machine guns, to shields, to homing missiles, to grenades, to swords...
Baiken of the game series Guilty Gear lost an arm and an eye. Despite it being Twenty Minutes into the Future, she didn't opt for a prosthetic. Instead she just filled that shirt sleeve with all kinds of stuff loosely attached to her shoulder, including a raking claw on a chain, and a fireworks launcher.
In the same vein as Barret above, Drachma from Skies of Arcadia has a wide variety of hands to fix to his mechanical arm. None are guns, but there's a variety of hooks, claws, prosthetics, blades, and what-have-you.
The old Megadrive/Genesis game Cyborg Justice gave you (and level bosses) the ability to rip arms off enemies and swap them with yours. The options included a circular saw, Rocket Punch and Lasers. Or you could just throw it back at them.
The main character in The Potter and the Mould possessed limited shapeshifting abilities which allowed him to turn his hand into all sorts of useful doohickies, from a crowbar to a screwdriver to an indestructable container for some pretty powerful acid.
The Mad Scientist in Exploitation Now has a robotic hand; there's some deconstruction in play since, due to the asymmetry in her weight being bad for her back, she has to take it off regularly, for instance in the shower. It comes with a vibrate function in addition to all the tools and weapons you'd expect.
Baron DeKlau in the Animated Adaptation of Ace Ventura, after having his hand bit off by rabid squirrels. He has a regular cosmetic hand designed to resemble a claw-fingered monster hand which can be swapped out for a variety of things such as a machete, a bullhorn, a grappling hook, a toilet paper roller and a metal pincher.
Cyborg has a cybernetic arm that transforms from hand to big cannon to whatever else seems particularly plot-relevant at the time. Then again, thanks to his cybernetics, he has more of a Swiss Army Half-Body, as writers tend to use his technological enhancements as a bit of a Deus ex Machina.
This also happens to Robin in "Fracture". "Larry" (who is from another dimension) accidentally changes Robin's arm while trying to heal it from a fracture. He fails, changing it from everything from a baseball bat to a chainsaw.
In The Transformers, many of the various characters have been seen withdrawing their hands into their forearms and extending new tools. Ironhide uses this especially often.
Lockdown of Transformers Animated can change out his arms and weaponry with other parts—from other Transformers. It's implied that the other people aren't always dead when he tears the pieces off.
In an homage to Enter the Dragon, Jackie Chan in Jackie Chan Adventures once fights a villain with different prosthetic hands. When going unarmed against the villain's weaponized hand, Jackie grabs other hands, which turn out to have rather mundane uses, like toothbrushes, instead of weapons.
Much like the Teen Titans example above, Cyborg in DC Super Friends has tons of gimmicks built into his body, but his lean more towards the Inspector Gadget end of the spectrum, with extending legs and a radar dish in his head.
A climber named Hugh Herr lost his lower legs to frostbite while climbing in 1982. By 1992, he's an inventor of human-assist devices, and one of his sets of prosthetic legs is built for climbing. They're about a foot longer than his natural set, and have specially-designed feet for finding footholds, to the point where his climbing buddies say he has an unfair advantage. He's now an Associate Professor at MIT.
Double amputee Aimee Mullins has about a dozen different prosthetic legs, including the special sprinting legs that made her famous when she ran track for Georgetown University, the legs that make her six inches taller than normal (yes, a double amputee has done runway modeling), and the clear plastic legs she wore in a movie. For a TED talk, she describes how she was talking to a class of children, and one girl posed the question: Why couldn't she get jet legs and fly?
Nadya Vessey lost both her legs when she was a child and had a prosthetic mermaid tail created for her so that she could swim.