Spiders are creepy. People with spider legs growing out of their body much more so. It does have a few perks, though. Characters who have an extra set of spidery appendages can often use them to scuttle about on walls and ceilings freely, jump great distances, and evenreach the jar of cookies hidden on the top shelf!
These spider limbs can also be used instead of walking on human legs, giving the character an extra meter or two of height, and almost seeming to float. These limbs can be of any make. Maybe they're Artificial Limbs, Combat Tentacles with Mundane Utility, or even inky tendrils born of the deepest darkness. Though for some reason they're always attached/growing out of the small of the back.
The limbs aren't usually prehensile, but some of the stranger variants are equipped with claws, hands, or cartoon strength static cling that allows the limbs to manipulate objects. Likewise, tentacle rather than spider based limbs can also be used as Tentacle Ropes.
Usually, this is exclusively a case of Bad Powers, Bad People due to the spider/octopus/tentacle factor. Still, there are cases of Dark Is Not Evil.
Not to be confused with Multi-Armed and Dangerous, but may overlap if the extra combat arms can also be used to scuttle around.
Medusa in Soul Eater utilises this as well, as part of her snake/arrow themed powers. She can balance on her tail and create many "snake" arrows that sprout out of her back, among other uses.
In Busou Renkin, Tokiko's busou renkin, the Valkyrie Skirt, takes on the form of four scythe-like blades that latch onto her legs and are controlled by nerve impulses through her skin. She can use them to move over difficult terrain, as well as climbing up walls and ceiling.
InuYashaBig Bad Naraku and his Combat Tentacles fall into this category. Though there is a certain element of Body Horror for sure, the underlying theme is definitely arachnid in nature as he was once a human named Onigumo (demon spider) and all of his incarnations manifest a spider on their backs.
Nakua from Princess Resurrection and Naqua-Den does this by growing turning her hair into spider legs to either attack someone or just doing something mundane.
Spider-Man in all his incarnations has a lot of instances of this trope. Firstly, there's arch enemy Doc Ock, then there's the Powered ArmourTony Stark gave Spider-Man in the Civil War, and there's also Midnight Sons rogue Spider-X, who had boney spider-limbs.
Pre-dating the Civil War Iron Spider armor, a possible future Spider-Man was shown to be a genius with Powered Armor using a similar system to Doc Ock's. Interestingly, the future Goblin serving as his nemesis had equivalent technology on her armor as well.
In Quest for the Fallen Star, the "Tenebrite" variant of Ill-creature is usually humanoid, but can morph its lower body into a set of spider legs.
While he doesn't possess any extra limbs, titular character The Arm from Nancy Farmer's The Ear, the Eye, and the Arm looks decidedly creepy thanks to his extra-long limbs. When he decides to ramp that effect up a bit, he unhinges them a little and crooks them in odd ways; he even uses this to scare some primitive tribesmen into thinking he's a boogie man.
In the Warhammer 40,000 novel series Soul Drinkers, Sarpedon's mutation is his legs turning into eight large spider-like legs. They prove to be extremely useful as the series progresses, and after Sarpedon is captured by the Imperial Fists, an Apothecary remarks that he finds Sarpedon's inhuman pelvic structure fascinating and looks forward to hanging it on his wall after Sarpedon's execution.
The Sci Fi Science episode "How to Build a Super Suit" had Dr. Michio Kaku's design, which included a pair of extensible robotic arms attached to the back and controlled by the user's brain. Dr. Kaku even mentioned Dr. Octopus by name. The arm control system is fully within the realm of current, or at most, Twenty Minutes into the Future science. A early version's been done with chimpanzees.
The demons who were Jasmine's previous worshipers in season 4 of Angel had lower bodies with four of these type of legs.
The Fiends (Neberu) in Demon The Fallen are associated with spiders (since they were the ones who "webbed" the entire universe together on the Fourth Day of Creation) and therefore prone to growing an extra pair of hands in their High-Torment Apocalyptic Forms.
In Exalted, the Terrestrial Circle Sorcery spell Lightning Spider gives the caster eight extra spiderlike limbs made of lightning!
While Alchemicals can achieve this with Manifold Transhuman Implants, the signature Apostate has modified himself into a full-on mechanical drider.
The Book of Vile Darkness sourcebook for Dungeons & Dragons has a spell that gives the caster a set of four giant spider legs. This lets them climb, move quicker, and raise their body out of the way of enemy attacks.
Around the same time, the Song and Silence sourcebook also featured the Fang of Lolth prestige class, which culminated with fully functional Half-Human(oid) Hybrid/Spider limbs.
In the Infernum D20 setting, this is the effect of the first mutation for the Chain of the Spider. The demon (or fallen angel, or corrupted human) sprouts a number of secondary appendages to give them eight limbs in total — of themselves, however, they simply make it easier for the character to climb; it takes the Chain of Multiple Arms mutations to make them strong enough to be used in combat or for tasks.
Occasionally found on tech-priests of the Imperium. Mechandrites, which are cybernetic tentacles tipped with various useful things (manipulators, specific tools, weapons, etc) are a de-facto cyber implant amongst their ranks.
Techmarines (Space Marines who also function as tech-priests) often have a large servo-harness mounted on their power armor with multiple grasping arms.
Andariel, the first boss from Diablo II features four spider legs growing from her back, but they're only used for attacking.
Ditto for Kerrigan, the Queen of Blades in Starcraft.
The Jedi Kazdan Paratus in Star Wars: The Force Unleashed is a rare heroic example...except he's gone senile and insane, making him all too creepy.
Laughing Octopus from Metal Gear Solid 4 is a rare exception to the small-of-the-back rule; her Combat Tentacles come from the top of her head, leaving her body dangling down when she uses them to move around.
Yagrum Bagarn, the last of the Dwemer race in The Elder Scrolls III: Morrowind became infected with the corprus disease. Unable to walk under his own power due to its ravages, he constructed a series of mechanized spider legs to carry his bloated form.
A couple of background characters are like this, apparently double amputees who use them as mobility prosthetics. Given that the whole backdrop is of a pseudo-Europe which has been in a state of low-level warfare for an indefinite period, It Makes Sense in Context.
Arachne of the Whateley Universe can manifest Spider Limbs along with her normal human limbs. She can also grow more spider-y in other ways when she chooses to, but she routinely is stuck with spider fangs that leak poison when she gets excited.
Irken's PAKs in Invader Zim can extrude these on command. He has a strikingly matter-of-fact handling of these (to-him) familiar tools. They are likely purely instinctual. There's one episode when Zim's right next to something blowing up, and when the smoke clears you can see his spider arms in the same position as his actual arms, trying to shield himself from the blast.
The Teen Titans cartoon often has Gizmo zipping around on four mechanical legs. Kinda like Doc Ock, but they only have about three joints each.
Does Fang count? He uses four giant spider legs to carry himself rapidly across any sort of terrain and to hoist himself out of reach of his enemies. Of course, they don't sprout from his back, per se, but from the abdomen of the mutant giant spider that has replaced his head. The humanoid body just sorta hangs there, waving around limply, as it's pulled around by the spider legs.
Darkwing Duck himself, in the episode "Aduckyphobia," received a radioactive spider bite that granted him two extra pairs of arms, though they seemed to have minds of their own (he got the hang of it when he learned they responded to verbal commands). The Spider-duck costume was pretty sweet.
Having the alternate forms of spiders, Tarantulus and Black Arachnia from Beast Wars naturally have these. Theirs come with machineguns.
Nidhiki in BIONICLE 2: Legends of Metru Nui has been mutated to have four arachnid legs.
Airachnid in Transformers Prime has six legs sticking out her back, and she switches from bipedial to... uh, hexapedal at will. She later gains a helicopter as her altmode.
In the two-part fourth season finale of Star Wars: The Clone Wars, Darth Maul returns after being presumed dead for many years with his lower half replaced by mechanical spider legs. He has these briefly before they are replaced by more human-like legs.