Envy can turn into literally anything. During a fight with Ling Yao, he turns his limbs into a sword and a snake.
In the 2003 anime version, Envy turns his arm into a black spike near the end. At another time, he copies Ed's arm and the sword trick.
Wrath in the 2003 animé had the ability to transmute his own body.
Naraku of Inu Yasha spontaneusly sprouts Combat Tentacles whenever he needs them, or morphs his hands into those to impale his enemies.
Elder Toguro of Yu Yu Hakusho loves turning his body parts into pointy things to stab people with.
Eve from Black Cat turns her limbs and hair into weapons, usually blades. Same for her Expy Yami-chan from To Love-Ru.
Chachamaru in Mahou Sensei Negima! can transform her forearms into blades or even guns. Judging from his ability to mimic other people exactly, Albireo can likely use his pactio item on his larynx. Evangeline also enjoys a good shape-shift here and there (claws are her favorite).
A subtrope is when someone bleeds themselves, and then morphs the blood into weapons. Seen in Flame of Recca and Black Cat. See Bloody Murder.
There was an episode of Pokémon involving a Ditto that eventually turned into a cannon for a Fastball Special. Now if it could only turn into an actual cannon in the games...
Soul Eater has a few shapeshifters able to transform parts of their bodies, but most have to change completely to be much use. Death Scythes can alter their weapon forms, which lets them make several of the same weapon part sprout from their body at once, making it a lot easier for them to fight solo this way.
Crona has the ability to use Ragnarok the Demon Sword. Although Ragnarok has its own soul and own personality, he was melted down and fused with Crona's blood, effectively meaning that it is a part of Crona.
Nube's Oni Hand in Hell Teacher Nube. He has to be careful when doing this, though, lest the hand overpower him and consume his entire body.
On Blood+, Solomon often uses his chevalier powers to turn his right hand into a sword blade.
Eve Neuschwanstein from Needless is a shapeshifter and likes to change costumes along with her arms into weapons.
Arashi Kishuu, from X1999, can draw a sword out of her hand whenever she needs it. There's also Yuzariha's dog/daemon Innuki, who can transform into a sword at will—and that's not even getting into the women who give birth to the holy swords... this series likes its swords.
Alissa Southerncross from Keroro Gunsou fights aliens with the help of her shape-shifting "daddy", Nebula.
The titular Parasytes - human cuisinarts who can strike faster than the eye can see.
Kanade from Angel Beats! had the abilities to change her arm into four different types of weapons.
Gajeel from Fairy Tail can change his limbs into a club, a spear or a sword. And a chainsaw.
Gavrill from Franken Fran can shapeshift her body partly or entirely into, among other things, deadly weapons. She mostly uses it to off people who are much weaker than her. When faced with either a lot of opponents or just one large one, she prefers to transform into a huge wolf-like creature and let out a real bloodbath.
Sandman, from Spider-Man's rogues gallery. Particularly fond of sand-hammers. Also Carnage, who is quite fond of axes.
The Skrulls have been known to do this on occasion. They favor battering-rams.
Hulkling from the Young Avengers, who is half-Kree and half-Skrull. He often gives himself armor, claws, and wings.
Ghost Rider 2099's left hand was a "morphable nanomer alloy" with "monomolecular" edges, capable of shredding pretty much anything it came into contact with when he shifted it into battle mode.
Mystique can do this with some effort, but usually doesn't because she still has organs and needs to remember where they are. Instead, for example, she'll shift most of the muscles of her upper body into her right arm just as she delivers a punch, thereby increasing its power. She's also been known to hide guns inside herself. At one point she even gave herself two heads and four arms to fight on two fronts at once.
Carnage (and to a lesser degree, the other symbiotes) is a perfect example, as he's famous for morphing his body into weapons, especially axes and other blades.
While Carnage and his ilk are seen most often using symbiote-weapons, Venom has been using one since it first bonded to Spider-Man: the web shooting. This ability is mostly limited by imagination; What-If and crossovers have shown other options, such as the Punisher getting the Venom symbiote and making guns with bullets of symbiote-stuff. Of course he also had a tremendous appetite, since the ammo had to come from SOM Ewhere...
Plastic Man uses his stretching power for shapeshifting more often than not, though he only rarely turns his limbs into weapons or anything like that. He's too creative. When Plas does make weapons, they're usually unconventional. In his famous battle against Fernus, he made his body into several hilarious weapons like sharks and cymbal monkeys. The kicker is that he transformed into these things all at the same time.
Martian Manhunter occasionally does this, as do his White Martian rivals. In fact, Manhunter is most likely to do this when fighting White Martians.
The liquid-metal T-1000 from Terminator 2 Judgment Day could turn its arms into blades — handy for killing people when it was imitating someone who wouldn't be carrying a firearm.
T-8000: Guns and explosives have chemicals, moving parts. It doesn't work that way. But it can form solid metal shapes.
John: Like what?
T-800: Knives and stabbing weapons.
John Carpenter's The Thing (1982) could spontaneously grow teeth, claws, arms, mouths, etc. anywhere on its body.
In the climactic battle on Who Framed Roger Rabbit?, Judge Doom, who is a toon in disguise, turns his hand first into an anvil and then a buzzsaw to attack Eddie Valiant.
Mystique did this in the first X-Menmovie with Wolverine's claws (which failed miserably against the real ones).
Transformers: Pretty much every weapon used by the titular species is formed from their limbs (except, for some reason, Optimus's gun). Other exceptions include Brawlvastator's over-shoulder missile launchers (his tank kibble) and Blackout's Chest Blaster.
Auton Mickey, in the Doctor Who episode "Rose," morphs his hands into, well, crude clubs. The other Autons get more prep time, and create much more effective weapons.
All the Final Form Rides in Kamen Rider Decade are Shapeshifter Weapons, with the exception of Den-O Momotaros and Double. The list includes independent creatures (Kuuga Gouram, Ryuki Dragreder, Hibiki Akanetaka, Zecter Kabuto) vehicles (Agito Tornader), and weapons (Faiz Blaster, Blade Blade, Kiva Arrow).
Obliterators in Warhammer 40000. These are genetically-engineered supermen who've caught a daemonic contagion that fuses them to their armour and gives them the ability to form a wide variety of guns from their bodies — up to and including giant laser and plasma cannons. Iron Warriors are particularly prone to catching this Obliterator Virus, even those who are not armoured Space Marines.
'Body Weaponry', psionic power introduced back from Eldritch Wizardy.
3.5 ed. has a Prestige Class, the Warshaper, basically devoted to this, allowing a character to use his extant shapechanging ability (gained from another class or racial feature) to give himself natural weapons and superstrength, along with a few other perks.
Forgotten Realms has Malaugrim — extraplanar villains trying to return to their old world, whose weapons of choice are spiky tentacles and suchlike. They also use these for surprise attacks after infiltrating in more innocent forms and growing a weapon when no one looks.
The Keepers, creepy Men in Black with malleable bodies, such that they can reshape their hands into clubs or knives.
GURPS: Magic has a whole set of spells called Partial Shapeshifting that allow the caster to transform body parts into enhanced versions of animal ones.
Exalted has several of these, but the two primary examples would be the Lunars and Alchemicals. The former does it spiritually and organically, while the latter does it technologically; the former is more flexible, but the latter can grow actual weapons up to and including magi-tech artillery pieces.
Huitzil in Darkstalkers has the rather unexplained ability to transform his arms and legs into various weapons and tools in combat. Also Rikuo, Lord Raptor, and Q-Bee, for reasons equally unexplained. Tron Bonne in Marvel Vs Capcom 2 has the same ability.
The MMO Warhammer Online features a Chaos class called the Marauder, who can turn his arms into giant blades, maces, and claws. Too bad they removed the female version, which would had have so much Fetish Fuel (let's see: an evil, muscular woman that can turn her arm into stuff)
The official explaination for male-only marauders is that they wanted them all to be bare-chested. Yes, all of them. But they wanted to avoid an explicit Mature rating; same reason Slaaneshi daemons (notoriously naked) are in short supply and covered up more-than-usual.
Emeralda of Xenogears. Her attacks include turning her legs into a drill, turning her arms into boomerangs, turning her body into a hammer, and splitting herself into several thin discs to slice her opponent with.
Mortal Kombat features Jax, who has a fatality that involves changing his cybernetic arms to blades and chopping up his foe.
Alex Mercer, protagonist of Prototype, is a good example. Whips, blades, claws, spikes that shoot up from the ground, tentacles, all this and more! Plus he can fly and ''eat'' people! The same goes for James Heller.
Algol, the final boss of Soulcalibur IV fights merged with Soul Edge and Soulcalibur, and can manifest blades or shields from both at will.
While Axl of Mega Man X doesn't literally change his limbs to turn into weapons, the same process and visual effects he uses to shapeshift is also used to create his arsenal of firearms, including his default Axl Bullets, which have the same color scheme as he does.
The visual effects also suggest that he can use his own mass to create weapons like Red's deathscythe.
Necrid of Soul Calibur has orbs of nonphysical origin for weapons, which he can transform into different weapons.
The Soul Edge itself is a literal shapeshifter weapon, despite the fact that it is usually depicted as a giant sword.
The eponymous weapon itself is this. While this was shown in II and III, it's in full force in Soulcalibur V by Elysium, Soul Calibur's will.
Vectorman's whole "orbot" schtick involved transforming his whole body into jets and drills and such.
Haudrale of Final Fantasy XI doesn't quite achieve transformation per se, but extruding a sword from the palm of his hand and then probably killing your entire party with it probably counts.
Traitor, Taiga's weapon in Duel Savior Destiny, has the ability to change its shape to that of numerous other weapons. Taiga's not really sure how it works, but he can turn what is normally a short sword into a longsword, a two handed axe, bombs, a hammer, a three sectioned staff, a trident and a power fist.
Averted with the shapeshifters in But I'm a Cat Person, who fight mostly with energy attacks and the natural weapons of their standard forms (fists, claws, etcetera). When Patrick used his arm as a club that one time, it stayed in its usual shape and was damaged accordingly.
Diamondhead from Ben 10 can turn his arm into a pointy crystal. The nanomachineblob Upgrade from the original series and the green-slime blob Goop from Ben 10 Alien Force have been seen molding their fists into various weapons as well.
The animated version of the Teen Titans character Plasmus is basically the poor man's Clayface. He eventually gains the power to spew acid sludge as well.
Madam Rouge, a member of the Quirky Miniboss Squad in the final season, also uses this- her powers are a combination rubber woman and more general Voluntary Shapeshifting, so doing things like turning her fingers into talons or elongating her whole body like a snake to strangle enemies are well-documented techniques.
In Static Shock, there is a recurring minor minion named Shiv, who can turn his arms into almost anything he can imagine, almost like a Green Lantern (but without the need for a ring.) As per his name, he prefers sharp things.
Jake of Adventure Time has incredible control over the size and density of every part of him, and utilises this to a great extent. At least once he even uses his jowls to beat on an unsuspecting foe (because seriously, who expects jowls to be used as a weapon?).