The Kamehameha is a powerful attack that took decades for Kamesennin (a.k.a. Master Roshi) to perfect. Goku saw Kamesennin perform the attack once, and pretty much got the hang of it after only a few times. A number of other characters, including Tenshinhan and Majin Buu, use this trope inconsistently. Cell had this ability as his schtick, being able to copy anything he sees just once. Thanks to his trait of assimilating DNA as well, he gains everyone's secondary powers as well, including Namekian super regeneration, Saiyan's intense resilience, and Frieza's ability to survive without an atmosphere.
Goku copied a number of techniques just by watching them used only once or twice like the After Image, Solar Flare, and the Hasshu-ken.
Everyone can copy Krillin's Destructo Disc, apparently. And as an extra "kick in the stones" to Krillin, some of those people can use it to greater success.
Vegeta copies the Z-Fighters' ability sense ki and suppress his own power level to avoid detection by scouters, despite this not even being something that can even be "observed" at all. Just learning that it's possible was enough for Vegeta to figure out how to do it himself in a matter of days at most, giving him a key advantage against Frieza's minions.
Baam in Tower of God is so talented in Shinsoo techniques that he easily can copy and apply them once he experiences them. In some cases he can improve the techniques with ease, though sometimes, special schools require a lot of improvement to even reach the original users level, as Love notices.
Claude in Star Ocean EX has this ability, though notably absent in the games.
While on a Not a Date, Maki in Airmaster gets the idea for a new attack move simply from watching a samurai movie. She later uses it without any practice, even though the samurai is using a sword and Maki is strictly legs and punches.
Ranma of Ranma ˝ is able to master the most esoteric combat moves within days, if not hours.
Angelic Layer prodigy Suzuhara Misaki is able to pick up her opponents' moves on the layer after seeing them only a few times, and she picks up moves performed by real people as well.
Kazuo Kiriyama in Battle Royale. He can mimic the skilled martial artist Sugimura after seeing him use the moves in combat a couple of times. Even when Sugimura gains the ability to use ki attacks, Kiriyama quickly copies this as well.
Ga-Rei Zero-: Kagura and Yomi fight a spirit which mirrors their moves exactly. Its effective enough to put Yomi out of action temporarily and give Kagura her first major injury.
In Naruto, Kakashi Hatake and the Uchiha clan have an explicit ability to copy other ninjas' moves with their Sharingan. Kakashi is known to screw with his opponents' minds by subtly hypnotising them into using techniques he already knows, making it look like he can copy their techniques before they even finish using them. The only limitation is that they have to be physically capable of using the moves they copy, meaning they can't copy moves based on bloodline traits or Tailed Beast abilities, but most anything else is fair game.
Subverted in that throughout the series so far Sasuke has only used to ability to imperfectly copy one Rock Lee's moves, Kakashi has only used moves that he copied from anonymous fighters before the series began and no other Uchiha has been seen copying moves at all. That said the Sharigan is a near Story Breaker Power for other reasons down the line.
One of the definitive powers of Ifurita, the "Demon God" android from El-Hazard: The Magnificent World, who can copy and improve on any attack used against her. (That she's pretty much indestructible just helps her pick up things.)
Several characters from The Prince of Tennis have an ability referred to as State of Self-Actualisation (Muga no Kyouchi), which allows them to subconsciously replicate any techniques ever witnessed. Some other characters, while not possessing Muga, can also duplicate their opponents' moves, giving them a mental edge.
While it didn't exactly happen overnight, Kid Samurai Yahiko nonetheless managed to duplicate one of Kenshin's signature techniques solely from observation — which is still fairly impressive, since Kenshin apparently went through several years of Training from Hell to master his Hiten Mitsurugi style.
Subverted big time at the end of the manga. Aoshi fights a member of a Quirky Miniboss Squad who specializes in copying the sword techniques of his enemies and gets a psychotic joy out of killing opponents with their own techniques. It appears to be working and the guy seems to have the edge on Aoshi until Aoshi suddenly kicks him right in the face and informs him that as a martial arts master using swords is only a part of his repertoire. (And the copycat can't copy Aoshi's kenpo skills). The angry copycat then tries to go through with killing Aoshi with his techniques, but Aoshi correctly anticipates the moves the copycat will use, comments on how dumb it is trying to kill him with moves that he knows inside and out, and then promptly breaks the copycat's swords and beats the crap out of him.
Big Bad Naraku, is able to consume other demons to steal their body parts and abilities.
Inuyasha's BFS is able to absorb new special abilities from some of the enemies he uses it to slay.
One of the two minions of the Big Bad in the first movie has this ability, which she uses to copy Miroku's Wind Tunnel. Apparently she thought it was simply a powerful attack and not a bona fide lethal curse that Miroku takes advantage of. She is defeated when she opens it wider to strengthen it and the predictable happens.
Mahou Sensei Negima! gives us Albireo Imma aka Ku:Nel Sanders. His Pactio card allows him to become anyone he's met for a few minutes, complete with all their abilities. However, he can't maintain someone stronger than him for more than that.
As of Chapters 239-241, Negi gets his Mile Vincula artifact via a Pactio with Princess Theodora (if you're confused, Negi is the partner, and Theo is the mage). Activating his card gives him a pocketbook with a copy of all of his Pactio partners' cards. Negi can use any of these copy cards to mimic his partners' abilities.
Jack Rakan demonstrates this ability too. He can use high-level Shinmeiryuu techniques from simply having seen Eishun perform them.
Tiger & Bunny Kaede can imitate the power of the last NEXT she touched, a very very important plot twist.
In the movie, Bonds of Fire, Kuwabara encounters a demon who can mimic any technique his opponent uses. After being matched move-for-move, Kuwabara tricks his enemy into copying a move that throws every single bit of spiritual energy at the target. The demon swiftly realizes he's just doomed himself, as while he can copy energy attacks, he can't copy his opponent's physical strength... and Kuwabara is far stronger than he is. Cue asskicking.
Gourmet, an enemy from the third season, had a Cannibalism Superpower. This unfortunately proved to be his downfall when he ate the Elder Toguro, who regenerated and took over Gourmet's body from within in the most horrible way possible.
Another example is Rando: he was a demon that could perform and even improve upon special abilities after seeing them used. Prior to his battle with Yusuke, he had already acquired up to 99 of them.
According to Decepticon rumor, Thrust of Transformers Armada can copy his opponent's fighting style just by watching him. He's never actually done this in canon, but since everyone's fighting styles seem to be either "run at enemy and grab his fists forcefully", "swing sword like baseball bat" or "shoot from a distance", he doesn't seem to have a reason to.
In Magical Girl Lyrical Nanoha A's, the final challenge — the manifested form of the Book of Darkness — not only has her own suite of nasty-sounding attacks, she's also got the ability to copy and enhance the attacks of anyone whose magic has been used to fuel her. Which, unfortunately, includes both Nanoha and Fate. Cue our heroes bravely running like hell to escape the blast-range of Nanoha's trademark Starlight Breaker. Hayate also makes use of this same power shortly thereafter to become an Instant Expert in magical combat.
Takeshi Yoshida, from the baseball manga Touch is able to imitate a pitcher's style perfectly just by watching them. He develops an impressive repertoire but is undone in the end by a lack of endurance.
In NEEDLESS, the protagonist and Big Bad, through the Zero Fragment, can memorize the series' huge amount of skills just by seeing it used or headbutting them. The villain's variant is pretty much the ultimate last word in Power Copying: he not only copies, he makes the copy stronger than the original.
In Fist of the North Star, Kenshiro reveals a martial arts ability that allows him to learn any special attack after witnessing it the first time.
Clare from Claymore is fairly adept at this, typically copying her allies rather than enemies. Roxanne of Love and Hate did it as well, but in a much more sinister way.
In One Piece, Luffy is able to replicate CP9's Soru technique after watching the technique a couple of times and adapting his Rubber Man powers to allow his body to perform the task. Much later Sanji successfully discovers how to perform their Moon Walk.
Blackbeard was revealed as being able to do this when he took the power of the Tremor-Tremor Fruit from the recently deceased Whitebeard. How he does it is unknown so far. He has a Devil Fruit power that allows him to absorb things, but the dialogue implies that it may have something to do with having a unique body structure. This became a more literal example of Mega Manning later on, as it was revealed that he can take the Devil Fruit powers of anyone that he's killed, though whether he is taking them for himself or giving them to his crew is unknown.
In Hunter × Hunter the leader of the Phantom Troupe can steal the Nen abilities of others by learning about them, asking questions, and knowing its name. The original user of that ability can then no longer use that ability again.
Ging is shown to be able to imitate other people's punching-based abilities after getting hit by them. Naturally, the drawbacks are that he has to take damage and that it's limited strictly to punches. In addition, people tend to only show their abilities as a last resort or to finish a battle quickly, so he rarely gets to copy in the first place. That being said, Ging refers to it only as a "talent" and not a power.
In Guyver, the zoanoid Aptom has a particularly terrifying version of this power. At first he could only poorly mimic other forms: admittedly he could do this after only looking at them, but his imitations were far inferior to the originals. Once he's reworked by one of the series' Big Bads, he gains the ability to perfectly replicate any form just by getting a sample of their DNA. Except he gathers samples by melding genetic matter with his own body, and he can do this while the target is still alive. This becomes Aptom's favored method of killing, and it is lethal to amazing degrees: at one point he has contact with another character for roughly 3 seconds before being turned into shreds of skin, then atomized. In that time, his DNA-merging trick had already seeped into the other character enough to let Aptom take over with all his abilities intact, plus those of the character. Oh, and if he wants he can just let the DNA replicating system seep into someone and overwrite them without absorbing their body, meaning he can replicate himself without limit so long as there are bodies around to infect. He can even do this to corpses.
In Gakuen Alice, Mikan and her mother Yuka both have the ability to turn other peoples abilities into gems and then absorb said gems into their body, gaining those powers.
Nanami from Katanagatari can learn any technique she sees once, and master it if she's seen it twice.
In Rozen Maiden when one doll defeats another, they not only gain the defeated doll's Rosa Mystica, but can also use their powers.
In the manga chapters of Saki set after the Prefecture finals, Yumeno Maho, one of the twokohai from Nodoka's previous school (she's the one with the ribbon), is revealed to have the power to copy any mahjong player, but she can only do it once per round per day each. She demonstrates this by using the styles of all the Kiyosumi players, culminating with her out-Rinshan Kaihoing Saki herself.
The protagonist of Medaka Box initially comes off as Always Someone Better, but it becomes apparent that every time she does lose to someone at something, she starts mimicking and equaling them. Post-Genre Shift, this turns out to be an actual superpower.
WanderingWarriorReina enters the Queen's Blade competition after dueling (and losing to) several of the other Queen's Blade competitors and incorporating their moves into her repertoire, taking her from rank amateur to world champion, and later defeats her friends and allies in the competition by taking on their goals and making them her own. As a finishing move.
In Saint Seiya: The Lost Canvas, Leo Regulus can fully understand how a technique works, copy it and execute it better than the original user just by seeing it.
Mega Man Megamix: Not as prominent as other versions, but Mega Man DOES copy powers here.
Kise of Kuroko No Basuke has the ability to copy any basketball technique he's seen and make it his own.
Actually, it's more like he uses Main/Awesomeness by Analysis to analyse moves and use them himself.
In Kinnikuman Stecasse King, a giant cassette player, inserts tapes from the Choujin Encyclopedia into his chest, gaining the abilities of whichever wrestler the tape covers. This proves his downfall when he tries to copy Kinnikuman — the tape he has is outdated, from back in the days when the big guy was a cowardly wimp.
Tachimukai from Inazuma Eleven is able to copy Endou's Hisatsu Techniques just by watching him, on TV!
Hana from Gate 7 is described as "Not" and therefore "Everything is taken in" meaning that zhe can adopt opponents' techniques as offense or defense as zhe likes. This serves to differentiate hir as much as associate hir to Chikahito who is "Not" as well
Several enemies in the first Sailor Moon anime had this ability, including the Gemini Warriors (copying Sailor Mars and Mercury), Techniclon (copying Sailor Jupiter), and Malachite (copying Sailor Jupiter, Venus, and Mars).
In Arata Kangatari, a Shou is allowed to use the powers of the Hayagami of other Shou who have submitted to them.
From Deadman Wonderland, there's Mockingbird: true to his codename, he can mimic the Branch of Sin of any Deadman whose blood he tastes, except for Ganta's special nuke shot.
In Anpanman, there's the Donburiman Trio. Because their heads are rice bowls (with the donburi contents acting as their power source), they can be refilled with different ingredients when their original ones are drained, which can change their personalities and powers, though they only last for one episode. Some examples include Tendonman being filled with tempura soba (giving him the samurai abilities that Katsuobushiman has), Katsudonman being filled with natto (giving him Nattoman's never-give-up additude and agility), and Kamameshidon having tuna sashimi in his head (giving him Tekkanomaki-chan's ninja-like skills).
It's similar to animal styled martial arts, They imagine themselves as the animals they're imitating, nya. In other words: The train is pulling into station four, please step behind the yellow line… KATAN KATAN, KATAN KATAN. Yes, this is the result of my special training. My goal is… a train!
Later, Miki will do the same, only she would imitate a bullet train.
In A Certain Magical Index, Rensa can use the powers of the Level 5 espers and any esper within a 200 meter radius, but one at a time. She cannot copy a power that she cannot understand, like Gunha Sogiita's. It is pointed out that she can't use a power as well as the original because esper powers require vast computational skills. She adds that if an esper dies, she can't copy their power anymore. When she tries to copy Touma's Imagine Breaker, her arm blows up.
Rogue from X-Men is possibly the best-known example in the world of comics. However, her power works on bare skin contact and works whether she wants it to or not and, in most cases, puts the "donor" into a coma that lasts as long as the powers are transferred to Rogue. In the Ultimate and movieverse version she even sucks the life out of the victim. This has been a constant source of angst, but since then she has gained better control so that touching people isn't always a near death sentence. There have been other X-Men examples over the years:
Synch from Generation X, though he possibly had the second-lamest power out of the group. As he couldn't keep any borrowed powers once their owner left his radius, there wasn't much he could do by himself and couldn't even fend off a pair of human bullies.
Several characters said he had the potential to be extremely powerful, more than once he was shown using borrowed powers better than their original owners, and he could use his power for lots of secondary effects like identifying and tracking other mutants. It's just that none of that helps him when faced with an entirely mundane threat like a bomb.
Gen-X's archenemy Emplate could this too, by feeding on a mutant's marrow via the mouths in his hands. Unlike most others, however, he must do this to survive. Basically, he's a mutant vampire. His victims gain the same mouths on their hands he has, and the same compulsion to feed, also similar to certain vampire stories.
Mimic, on the other hand, need only be in the proximity of another superhuman. He gets a bonus, too: The main Marvel Universe Mimic has permanently taken on the powers of the original five X-Men in addition to this, and his Parallel Universe double, the Exiles version, can hold any five powers at a time (though at half the original power level each).
Pandemic injected himself with a virus that would copy mutant powers. He is defeated when Sabretooth has him gain his Healing Factor.
Hope Summers, mutant messiah, has this power along with the ability to influence the mutants around her.
Taskmaster, of Marvel Comics, possesses photographic reflexes. He can do anything after seeing it performed once, with exactly as much skill as the person he watched had in the task. Up to and including Chow Yun-Fat◊. However, he gains the ability to use it, not their powers. Meaning he can do the same flips as Spider-man, but he'll never be able to leap off a sheer wall. However, he can watch a scene in fast-forward and perform it at double-speed for a (very) limited time.
Even more so, he uses his ability of reflecting an enemy's moves to predict what they would do next. Which got him absolutely nothing when Deadpool kicked his ass with the power of dance and randomness.
Cassandra Cain from DC Comics does as well as an extension of her ability to read body language. It's not so much a superpower as she's just so good at reading enemies and martial arts in general that one look is all it takes, and she can learn entire styles perfectly in minutes.
As does Ryan Tabbot, from Gold Digger, though his version is a bit worse in that while he can copy any move he sees, he still has to figure out how best to use it, so some highly specialized moves would get him in more trouble than they're worth if he used them.
The Super-Adaptoid can copy the powers of any superhuman it fights. In the Marvel Adventures series, it could also think like its target - which meant once it had copied the Avengers, it no longer wanted to fight, seeing them as the good guys now.
In the Guardians of the Galaxy future of the Marvel Universe, The Protege has this ability for all inherent abilities, with no restrictions and can combine them. Needless to say, she quickly becomes the most powerful being in her universe. Eventually, she tries to use these to become the new One-Above-All before getting smacked down by the Living Tribunal.
Over in the DCU, we have the android Amazo (created 6 years before the Super-Adaptoid), who has this ability. Just to make things tougher for the heroes, sometimes he comes with powers pre-programmed, and then copies some more. Amazo's power is taken to the ultimate level in the DCAU.
Also in the DCU, we have Libra. Who first appeared as a minor Justice League villain in the 70's, before Grant Morrison got hold of him and had him as one of the main villains of Final Crisis, being pretty much responsible for all the evil not committed by Darkseid personally, including for the almost death of Lois Lane.
Also also in the DCU, the Parasitic Teutons of Assimilation, or PTA. WASPs engineered with the ability to copy any superpower they see in action. Their exact origin is a mystery, but be honest: would you really want to know?
There's the relatively minor villain Paragon who can copy the powers of everyone near him at heightened levels. So if you have Super Strength, he gains greater Super Strength. The only reason he hasn't taken over the world is because he only retains his copied powers while the source of them is nearby.
Mega Man of Archie Comics. It's pretty accurate to games, though also a source of angst as Rock is inherently a pacifist who is gaining ever more powerful weapons.
The Absorbing Man, a longtime opponent of The Mighty Thor, is a villainous example. He can absorb the physical characteristics of anything he touches, whether inanimate substances like steel and concrete or even other people like The Incredible Hercules. This ability makes him a dangerous opponent, but it also means he often absorbs the weaknesses of whatever he's touched. Most of his defeats come from heroes who exploit this.
Duplicate Boy from the Legion of Substitute Heroes is unable to join the Legion of Super Heroes because he can only duplicate the powers other heroes have, and each Legion member must have a unique power. However, his ability is shown to have limits when he tries to duplicate the powers of an immensely powerful foe and is unable to, instead fainting from the effort.
The Midnight Sons crossoverMidnight Massacre has Blade of the Nightstalkers, in his quest against all occult, reading a certain page from Darkhold and turning into Switchblade the Demogorge, who obtains powers of anyone he kills. This power proves to be his undoing when kills Ghost Rider, and feels the power of his Penance Stare as he obtains it, buying enoug time for Prof. Hastings of the Darkhold Redeemers to fix things up.
In the Alternate UniverseTwilight fic Luminosity, Addy can, upon physical contact, perfectly replicate the powers of any person with special abilities (called "witches"). She can only hold one at a time, however.
The gloriously bonkers Neon Genesis EvangelionCrack FicA Little Angel On My Shoulder has the Instrumemory system, which is a bit different from most examples of this trope since it involves the Shoulder Angels (basically pint-sized ghosts of the canon Angels) temporarily merging with the Eva to allow it to borrow the Angel in question's abilities.
The Superwomen of Eva series' initial (and unlike most of the others not based on a DC or Marvel character but an original idea) heroine Spirit (created by Mike313 has the ability to absorb angel DNA to acquire their powers in addition to her Flying Brick and regenerative abilities. However, unusually for this trope, Hikari does not make use of this power much aside from her initial acquisition of Sachiel's ability to fire cross shaped energy blasts, which she has in every appearance as one of her trademark attacks (which is a lot; Spirit is probably the series' most frequent guest hero, appearing in multiple stories, by all 3 authors). Which means she's somewhat lacking in power compared to Her evil counterpart, Rei III who has the power of many angels. She does, however, also take Bardiel's ability to take over Nerv weapons which she uses to seal Rei III in bakelite in her title story, and in orionpax09's Lilith's Herald (Rei as Marvel's Silver Surfer) Hikari accidentally takes Sahaquiel's ability to fire explosive projectiles of its own mass (in Spirit's case her feathers).
In the latest chapter of Lilith's Herald Spirit finally willingly makes use of this power and copies the powers of most of the previous Angels fought thus far through DNA samples Rei obtained for her, making her similar in power to her final foe in the Spirit story, although it manifested differently (that enemy mostly used its hands to wield its power while Spirit creates energy whips from her hair, and fires Ramiel's focused energy beams from her eyes). Conveniently the Angels that had no powers like the 6th (which was a big whale thing that just crashed into stuff) or the 8th (which just sat in a volcano doing nothing) were the ones Rei couldn't sample.
In the Pony POV Series, it's revealed that Twilight's ability to learn any spell after witnessing it is because this is a special ability of the Element of Magic. Since Trixie is another Element of Magic, she's able to do likewise.
In RainbowDoubleDash's Lunaverse, the Element of Magic there, Trixie Lulamoon is able to learn spells "by ear"; that is, she learns how to cast spells by watching other ponies do it. Unfortunately, she's not very good at learning spells via spellbooks.
In Opening Dangerous Gates, the demon twins can copy their opponent's power level and skills, but it takes some time to work. Since they started out weaker than him, Grimmjow got cocky, giving them enough time to match his power. He beats them anyway.
In RWBY Reckoning, Darrel's Semblance is this. By touching an ally or enemy, he can copy their unique ability (i.e. Ruby's Super Speed, Blake's Flash Step), and use it to his own desire, though he can only do it once, and can only hold one power at a time.
Po from Kung Fu Panda learns the Wuxi finger hold, and a technique to manipulate water droplets after watching them once. His master Shifu is a little bit jealous.
In the movie Limitless, the main character becomes a master martial artist by remembering and copying the martial arts movies he has seen.
Zen in Chocolate is an autistic girl who learned martial arts, mostly Kung Fu, from watching TV. Late in the movie she's stymied by a fighter using a drunken monkey technique, and has to take a few licks before she adapts the form.
All of the competent channelers in The Wheel of Time series do this on a regular basis. Some of them even do it to each other when they intuitively leap to new things (specific example: when Egwene ties off a weave but can't figure out how she did it, then Elayne learns from her, and Egwene watches Elayne do it so that she can repeat it).
Protagonist Lire of Deadly Remains lives in a world where magic and Psychic Powers are real and publicly known. She normally has the power of Psychometry (read the thoughts of a person who handled an object). By touching the remains of three psychic murder victims (to read them), she gains the powers of telekinesis, pyrokinesis, and cryokinesis. With practice, she gains the ability to combine the abilities.
Unique among the Thirteen Orphans in Breaking the Wall, Albert Yu, representing the Cat, is able to temporarily mimic the status and abilities of any one of the other twelve traditional places on the Eastern Zodiac.
Baby Prudence of The Parasol Protectorate combines this with a form of Power Nullifier. Her touch reverts werewolves and vampires to their mortal states and allows her to use their powers until the sun rises or she is touched by a Preternatural, at which point she and her victim return to their previous states. The effects are also connected to her current target; If she strays too far or the victim dies, she returns to her natural form.
General Grievous' mastery of lightsaber combat is explained in the novelization of Star Wars: Revenge of the Sith. The computer in his brain analyzes his opponent's fighting style and allows him to utilize it himself, even Mace Windu's unique Vaapad. This ends biting him in the ass when the Genre Savvy Mace Windu sicks Obi-Wan on him, as his foe is the master of an incredibly basic style focused on blocking everything and is at least knowledgeable in the other styles, resulting in Grievous' retreat after Obi-Wan negated his attacks until he grew bored and cut two of his wrists.
In The Legend of Sun Knight, Creus, the Sun Knight in question, is able to learn any magic spell just by watching it. Including Necromancy. Which the Sun Knight is not supposed to know. Oops.
Sylar and Peter Petrelli from Heroes both are able to acquire new powers from other evolved humans. Peter just has to remember how that hero made him feel; Sylar's method is... a little messier.
In the graphic novels, one character, Linda, has a power that allows her to see the auras of people around her, and seemingly absorb them. Not only does it happen to absorb superhuman powers, but it allows her to kill them as well. Nifty. Unfortunately, she got a bridge dropped on her in her very first appearance, and didn't do anything too spectacular in the two other comics she appeared in (thanks to Anachronic Order). Ah, well.
Like Taskmaster, Monica Dawson's power is to be able to perform any physical feat she sees, from cutting a tomato into a rose like the TV chef to pulling wirework-style Kung Fu from action flicks. She receives a Blackberry with videos of various skills (plumbing, fighting, flying airplanes, etc) that she can watch to copy any time she needs to.
And now Arthur Petrelli, the Big Bad of Season 3: Villains, is revealed to have the ability to steal powers via touch, acquiring new abilities while depowering the original owners. It seems the apple doesn't fall very far from the tree in the Petrelli family.
This was the main ability of Warlocks, the Monster of the Week enemies from the show's early seasons, which was their primary motive for hunting and killing witches. Despite their potential for leveling up, they were pretty much at the bottom of the Sorting Algorithm of Evil because they were mostly too weak to kill anything with substantial power (such as a demon).
Zankou, the second-most-powerful demon in existence and the Big Bad of Season 7, demonstrated the ability to absorb the powers of those he kills, but only really used this ability once.
Cole, after being sent to the demon afterlife, managed to avoid disintegration and found he could absorb the lingering powers left behind by all the demons sent there to be destroyed. Eventually he absorbed enough powers to kill the serpent that devoured the souls sent there and was able to return to earth, with a ridiculously large array of random abilities.
Kamen Rider Raia, from Kamen Rider Ryuki has an Advent Card that allows him to copy the weapon of another Rider.
The Riders in Kamen Rider Blade have access to the Rouze Card system. This allows them to seal an Undead into a card and use this card to utilize the power of said monster to a limited extent. IE, using a card of a lightning-wielding deer monster allows the Rider to shoot a blast of lightning himself. Played even straighter with the Joker Undead, who is able to use aforementioned cards to gain the form of the sealed Undead, including all of its powers and abilities. In this case, borrowing these forms overwrites the identity of the Joker Undead, as using the form of the Human Undead causes him to become Aikawa Hajime.
The Cassisworm Gladius from Kamen Rider Kabuto has the ability to absorb the finisher attacks from the Kamen Riders, enabling him to use stronger versions of said attacks. However, he can't use this skill when he is simultaneously hit with multiple finishers. To a lesser extent, the Worms are able to fully copy human beings, including memories and skills.
Kamen Rider OOO uses a similar concept, with the main character having to take Core Medals from the Big Bads this series, the Greed, in order to use the powers contained in them. MostKamen Riders have some element of Phlebotinum Rebel, but OOO is probably the only one whose powers are chunks of his villains instead of merely made by them.
Kamen Rider Fourze offers a villainous example in the form of the Leo Zodiarts, who can change into any of the other 11 Horoscope Zodiarts by using their Astro Switches.
Kamen Rider Gaim's Super Mode Kiwami Arms, unlike the other Rider forms in this series, doesn't have its own unique weapon. Instead, it has the ability to use everyone else's weapons at will — and can even use them better than their original owners, primarily by Dual Wielding (either duplicate copies of the same weapon, or two different weapons at the same time).
And then Super Sentai got in on the act. In Kaizoku Sentai Gokaiger, the heroes have a stash of Ranger Keys, which unlock the powers of previous Sentai teams. Exactly like Decade and his cards.
Star Trek: The Borg have the ability to assimilate technology and knowledge from other species. It is at the very core of their philosophy.
Resistance is futile.
In one episode of Stargate SG-1, the planet of the week features a civilization with nanites in their brain. They have child prodigies learn everything they can about a certain subject (one character is an 11-year-old expert on Naquadah reactors). When they reach a certain age, the nanites are harvested and distributed to the other members of the species, and suddenly everyone with a nanite has that knowledge. The downside is that removing the nanites causes their original possessor to regress to an infantile state and adding new nanites doesn't work.
In Ultraman Ginga Hikaru can transform into previous Ultra Heroes and Monsters, by scanning the Spark Dolls with his Ginga Spark. Interestingly enough the Spark Dolls are actually the sealed forms of the previously mentioned Heroes and Monsters instead of a replica of their powers.
The title character of the short-lived 80s show Automan could duplicate any skill he'd seen used at least once.
Every major Superhero RPG ever. DC Heroes (the eighties/nineties version from Mayfair Games) had both Mimic, which copied powers, and Power Drain, which gave them to the user while removing them from the target. Champions characters with this ability were and still are most typically built with a Power Pool (a point reserve to be assigned to actual powers at appropriate times) with appropriate special effects and limitations. Marvel Super Heroes had relevant powers in its various editions and sourcebooks...the list goes on.
In Vampire: The Masquerade, a vampire can steal another vampire's power by devouring the vampire's soul. The game considers this act, called Diablerie, to be a Very, Very Bad Thing, especially in the eyes of the Camarilla... Though they mostly dislike it for the precedent it sets.
Werewolf: The Apocalypse gives the Ragabash (sneaky, trickster werewolves) the Thieving Talons of the Magpie Gift, which allows them to temporarily copy any supernatural power they witness.
The successor game, Werewolf: The Forsaken, gives the Irraka (also sneaky werewolves) a Milestone Gift that allows them to do the same thing permanently, as long as they witness the power on multiple occasions and make the right expenditures of Willpower.
In Exalted, Eclipse Caste Solars and Moonshadow Caste Abyssals can learn the Charms of other types of Exalted, as well as Charms unique to spirits. The catch is they have to be taught the Charm by an Exalt of that type (or Spirit) who knows it.
Apostate Alchemicals have a Charm that physically tears off the Charms of other Alchemicals and bolts the stolen device to their own form. This cruel enhancement, known as Flesh and Steel Transference Mechanism, was nicknamed "Mega Man Method" during development.
There is a high-level charm that allows you to copy every power that a being possess. You can even use powers that you normally can't ever learn, such as Sidereal Martial Arts or Lunar Knacks. The bad news is that that charm belongs to The Ebon Dragon.
In the Dungeons & Dragons book Serpent Kingdoms, the Sarrukh has an ability called "Manipulate Form" which can permanently imbue any reptilian creature with its own physical features and powers. This forms the basis of the infamous Game Breaker character "Pun-Pun", which can copy everything in the universe at the same time, with the power increased as much as he wants. By some interpretations of the rules he can even make up things to copy.
Oh, and this can be done at 1st level (although only in the Forgotten Realms universe). It is an important lesson in what happens when you take the rules literally.
The Tyranids of Warhammer 40,000 do something similar to Power Copying. When they turn their opponents into the raw materials to make more Tyranids, they also absorb their information and occasionally their traits; these are then used by the hive fleet to make it easier to counter those tactics and abilities. Some sources also hint that some varieties of Tyranid were "invented" using the DNA of certain foes, such as the psychic Zoanthropes (believed to have come from Eldar DNA) and the durable Tyrant Guard (supposedly created with Space Marine DNA).
The Kroot also do something like this. They consume their slain enemies, and any useful traits are incorporated into the Kroot's DNA (which is mostly blank apart from their natural traits). Over a few generations, the traits gained from consumed enemies become a natural part of the Kroot's biology.
The Kroot Shapers are shamans within tribes that direct what they eat, to ensure only good traits are retained and not detrimental ones. This came after one section of kroot decided it would be good to assimilate canine DNA, and getting trapped in an evolutionary dead end. The current Kroot themselves are actually not the base race, they use to be a type of scavenging bird before consuming a humanoid race.
The chess variantPlunder Chess allows pieces that capture other pieces to use the captured piece's move once.
There's also the gruesome Pathfinder spell, Blood Transcription, which requires you to "consume" a pint of blood from a dead spellcaster, allowing you to learn one of their spells, provided your class can learn it. Needless to say, this spell comes with an 'evil' descriptor.
In Magic: The Gathering, there's a number of creatures that copy individual creatures, but for simple sharing of powers, Experiment Kraj and to a lesser extent, all Simic cards share their abilities through Cytoplasts.
The Mask of Kindred in BIONICLE. It allows the wearer to copy the abilities of animals sharing the general environment with them (so land animals if the user is on land, sea animals if they're diving, etc).
Also, the Mask of Emulation, which allows the wearer to use any power he can watch someone else using.
One minor character has the ability to become the master of any activity he sees being performed. This doesn't encompass powers, but it does let him, say, become proficient in the melee fighting style of his opponent. And be better at it.
Vezok can absorb powers used against him. He can also combine them, though it's unclear how long he can keep one power.
Krahka can change her form at will, and unlike other shapeshifters she can also use the powers of the copied form. Krahka can combine several forms and powers at once, to the point that she can create a Toa Seal on her own as long as she copies six different Elemental Powers. As with Vezok, the exact limits of this ability is unclear.
The Mega Man action figures (based on the Ruby-Spears animated series,) has this as its main feature. Every robot figure in the line can replace one hand with a spring loaded arm cannon (though only a couple actually COME with an arm cannon) and all the various weapons are completely interchangeable, which not only allows Mega Man himself to do his thing, but many other wild combinations—like having Cut Man fire plasma shots, bombs, or even Guts Man's fire hydrant from his head.
Mega Man, the former Trope Namer, has the ability to utilize the weapons of the various Robot Masters that serve as the bosses for the series—though there is some occasional alteration for the sake of gameplay or other compatibility issues. Almost all his Expies and Alternate Universe counterparts either have this ability direct or use something based off the same concept.
Similar to the above, the Battle Network series introduces battlechips which are most commonly acquired by defeating an enemy and most commonly involve using that enemy's attack. What's notable is that this is a basic ability of all characters and enemies but for the most part all characters except the main character prefer to use abilities directly related to themselves rather than the wider array of battle chips they could have.
Then taken further with the various soul systems introduced in later games where Mega Man gains the ability to mimic the appearance and properties of other characters in addition to using battlechips.
Bass.EXE has his Get ability, which he uses to absorb data to gain more power and abilities, not unlike his Classic counterpart. Which is inherited in Mega Man Star Force by the Geo Stelar/Omega Xis fusion Mega Man. If you curbstomp mooks with sufficient skill, you gain a battle card representing their attack. Usually you can use it better than they can - in 2, the Mettenna's GrndWave1, for example, does a lot more damage in Geo's hands and can hit multiple targets.
Kirby, the eponymous star of his own Nintendo video game series and the anime Kirby of the Stars, copies the power of any enemy he can successfully eat, except for in the first game.
This also carried over to Super Smash Bros., although he can't really eat his enemies. He also only copies one move of the inhaled opponent, which replaces his Inhale move until you decide to drop the ability by taunting or if you take enough damage from the opponent(s).
In Kirby Super Star, he can even use the ability to create a "Helper", an ally with similar abilities to the enemy that commonly has the skill (often a Palette Swap). The helpers themselves has a bit of Power Copying; if one's health is reduced to zero, they begin to glow before exploding. In this state, they simply need to touch an enemy to turn into that enemy's respective helper and completely restore their health to boot. One of the abilities also allows Kirby to copy powers by scanning the enemy rather than inhaling them. This is rather pointless for Kirby, but gives the Helper even more freedom.
Played with in Milky Way Wishes, where Kirby cannot copy enemies by eating them. Kirby has to find them in capsules hidden throughout the levels, which lets Kirby can select that ability at any time from the pause screen, much like Mega Man. The "scanning" version of Copy still works, which makes it much more useful - if you know where to find it.
Some games in the Heroes of Might and Magic series feature the skill Eagle Eye, which allows the hero to learn spells by watching the enemy hero cast them in battle.
Considering that since you learn them from enemy heroes and are likely to conquer the towns they learned them from, skilled players more or less ignore it for better skills such as Wisdom, which is needed to learn the higher level spells to begin with. And generally the highest level spells you could get was level 4.
In V, this also works against spellcaster creatures, as they use the same spells. However, in neither case Eagle Eye allows heroes to learn spells that require the respective magic skill (3rd to 5th cycle) or abilities unique to the hero class (Barbarian heroes, who use War Cries instead of magic, can learn War Cries but not magic with a similar skill).
Metroid: Samus's Power Suit has the ability to integrate foreign technology if it encounters something useful. To wit:
In Metroid Prime, Samus gets her Plasma Beam by copying what was apparently originally a beam used by the Space Pirates for mining.
Samus gets a missile expansion from the Dark Missile Trooper in Metroid Prime 2. The Ing item guardians are a double example; after they steal your equipment at the beginning, you can see them using it when they fight you, then you kill them to get it back.
In the third Prime, the Nova Beam is collected after destroying a mining platform that was using it.
Also in Corruption, defeating each of the corrupted bounty hunters yields an upgrade based on their powers. Rundas gives ice missiles, Ghor gives the Plasma Beam (which is explicitly stated to be his main weapon), and Gandrayda drops the grapple voltage.
Metroid Fusion has this trope with nearly every boss. You get the Morph Ball from the first boss that can curl up into a ball and charge at you, the Gravity Suit comes from an enemy that can manipulate gravity, the Plasma Beam is acquired after you kill a plant that shoots lasers at you...
Because these bosses have once possessed her power armor.
In Metroid Prime, the Space Pirates invert this on Samus. They reverse-engineer Samus' beams, and unleash troopers outfitted with these counterparts at her.
The protagonist of inFAMOUS: Second Son , Delsin, has the ability to take on the powers of other Conduits.
In the Disgaea series, characters are able to learn magic, weapon skills, and abilities from others on their team through different methods, depending on the game.
In the firsttwo games, only magic could be learned via the master and student system, which allows a character to use the spells of an adjacent unit if they were the ones who created them. The spell would be learned permanently with enough uses.
Disgaea 3: Absence of Justice introduces the Class World, where a character who's leading a school club can learn the magic, weapon skills, and abilities (Or "evilities", as they're called in-game) possessed by any characters in the same club, albeit for a much higher price then what they purchased them for. It also gives Yukimaru a unique primary evility that makes her to copy the primary evility of a target she kills.
Dark Hero Days adds the Magichange 2 ability, which allows a humanoid character to use the special attacks of the monster who performs it on them.
Many variations on Blue Mage (incidentally the same color as the former Trope Namer) from the Final Fantasy series of games work like this. The precise variant varies from game to game — sometimes, they have to be hit by the ability, and in others they merely have to be in the party when the ability is used, and some have to do something special related to killing the enemy to do it (Final Fantasy X has you use an Absorb-like spell, while Final Fantasy IX has you eating and cooking enemies). Final Fantasy VI specifies that the Blue Mage has to see the spell being cast, so he can't learn anything if he's Blinded. In a variation on the theme, Summoners in the series often must defeat prospective summons in combat before being allowed to summon them.
Another variant is the Mime, who can repeat the last action taken by an ally, no matter what it was, with no MP or item cost.
This is how Ramza learns the Ultima spell in Final Fantasy Tactics, having to get hit first. You need to be a Squire for that battle, and be careful not to be turned into a frog or charmed instead. Likewise, summoners learn the hidden ultimate summon in this manner. There are some other spells that can work like this (the level 4 spells, most high end summons), but those can be learned normally, which is generally less troublesome.
Enemy Skill materia in Final Fantasy VII also lets you learn a, well, enemy's skill that is used on you. Also the Mime materia, which when equipped lets you copy what ever move was just used. This included summons and Limit Breaks, although the latter wouldn't work if it wasn't yours.
Gau's Rage ability from Final Fantasy VI, which he can use each individual enemy abilities which can be learned at his home habitat in the Veldt.
Final Fantasy X's Blitzball mini game allows the players of your team to mark and learn new abilities from the players of the opponent team.
Shanoa in Castlevania: Order of Ecclesia is able to perform power-copying feats by absorbing magical glyphs. In several cases, she can steal a monster's magic in the middle of battle. She even kills a boss this way.
Don't forget Dmitrii from Dawn of Sorrow, who can copy Soma's magic souls by being hurt by them... However, it makes him a pathetically easy boss if you know what soul to use on him - he will copy that attack, no matter what it is, so making him use a weak and easily avoidable attack makes the battle a cakewalk. Later on, it turns out that by having his soul absorbed by Soma, Dmitri was able to copy Soma's power of domination, at which point he becomes the game's Bigger Bad.
And of course we have Soma himself who has the ability to steal the soul and abilities of ANY ENEMY IN THE GAME (That's saying something in a Castlevania game). This allows you to gain an attack or special property for as many as there are different enemies in the game (with some extras).
Charlotte, from Portrait of Ruin can learn certain enemy attacks if she absorbs them using her Book of Binding, which improves the books' standard abilities as well as allowing her to cast those attacks back.
In X-Men vs. Street Fighter, Rogue's power-stealing abilities are represented as a special move that allows her to copy one of her opponents' special moves by kissing them. In Marvel vs. Capcom 2: New Age Of Heroes, this power couldn't work because there were 56 characters total, so instead it gives Rogue a bonus to various stats like Attack, Defense, Speed, or Health.
The game to which Samurai Legend is a Spiritual Sequel, Brave Fencer Musashi, also allowed Musashi to copy enemy moves by absorbing the enemies into his shortsword, the aptly-named Fusion. He could only hold one at once, though, as opposed to Samurai Legend's version being able to learn as many as he pleases.
Mirror Move and Copycat mimic the last move used by an opponent, while Me First can mimic an opponent's move before they use it.
Mimic teaches the user one of the opponent's moves for the duration of the current battle, allowing them to use it up to five times.(though the gen IV Mimic glitch could make it permanent.)
Sketch teaches the user one of the opponent's moves permanently, and is exclusive to the Pokemon Smeargle, who learns a new Sketch every ten levels. With enough time and effort, a player can potentially teach Smeargle any move in the game,(except Struggle or Chatter) although Smeargle is by no means a strong fighter. A male Smeargle can thus be used to breed any desirable move onto any baby Pokemon in a compatible breeding group.
Ditto can use the moves of whatever it transforms into for that battle.
The ability Trace (used by the Porygon and Ralts lines, as well as Mega Alakazam) copies an opponent's ability when the user enters the battle. The Porygon line also has Conversion to copy the opponent's type during the first generation games.
In Sonic Adventure 2, Sonic is able to use Shadow's ability of Chaos Control after seeing him use it just once.note It's implied that Sonic could always use Chaos Control, but he wasn't aware and/or didn't know how to.
Sonic Chronicles gives us the Gizoids, of which Emerl was the most powerful. Though unlike Emerl, who doesn't seem to have a limit in what he can copy, the Gizoid Mooks are only capable of copying one or two special attacks from the main characters.
The final boss of Sonic Heroes copies Chaos Control from Shadow, Chaos' ability to manipulate water (Though in this case, he uses metal) from Froggy and Cheese and everyone elses' own powers.
To a lesser extent, the Chao. They can absorb the power and some physical traits of any tiny animal presented to them. They can mimic the physical traits of their owners, to some extent- the Sonic and Shadow chao being the prime offenders.
Metal Sonic from the second Sonic Rivals game has no Signature Move of his own - instead, he copies it from the opponent he's currently facing. In case he's alone or two Metal Sonics are fighting each other, it defaults to Sonic Boom.
Jade Empire seems to teach new fighting styles to the player character this way.
Revolver Ocelot from the Metal Gear series falls squarely into this trope. According to the supplemental material, after merely witnessing a tactic on the battlefield, he can them use said tactic thereafter. This certainly explains his ludicrous skills with revolvers, but he is noticeably clumsier as his younger self in Metal Gear Solid 3, where Snake calls him out on using a tactic he's only just seen in battle.
At the end of the game he fights Snake hand-to-hand, and uses CQC moves seconds after Snake uses them on him.
All main Metal Gear games, starting from Metal Gear Solid 3, feature this trope to some extent. Stamina killing a boss in MGS3 gives you a camo pattern that provides a certain bonus (for example, The Fear's pattern increases Snake's camo index at the cost of a stamina drain, while the Fury's halves all fire and explosive damage.) In Metal Gear Solid 4, beating a member of the Beauty and the Beast unit gives Snake a new weapon (in Laughing Octopus' case, Face Camo) while in Peace Walker, beating an AI weapon several times unlocks a weapon to be placed on Metal Gear ZEKE (the Chrysalis' railgun is required, though.) Lastly, in Rising, beating a member of the Winds of Destruction unlocks a copy of their weapon for purchase. Completing the game will also give access to Samuel Rodrigues' HF Blade.
In the Legacy of Kain series, Raziel and Kain steal their enemies' abilities by devouring their souls/blood in Soul Reaver and Blood Omen 2, respectively. One review of Soul Reaver actually referred to Raziel as "a sort of demonic Mega Man" for this reason. Interestingly, Raziel actually keeps almost all of the abilities he gains this way in his first appearance through his next two.
Shirou of Fate/stay night ( and by extension, Archer) is a rather Inept Mage in most aspects, but he eventually discovers that he excels in a special brand of Projection magic, able to reconstruct any sword-type weapon he sees as well as the original's owner's skill in using it. In the final route of the game, True Assassin seems to exhibit a form of this, as well, gaining form and intelligence by eating the hearts/bodies of his opponents.
In the obscure cyberpunk FPS game Cyber Mage by Origin, one of the ways the titular character can gain new Darklight spells is by allowing them to hit him, so that the crystal implanted in his forehead can replicate the spell. You can do this either by letting enemies cast the spell on you, or by finding Icons that cast the spell and shooting one into a nearby wall.
The only way to learn new attacks in Digimon World for the PSX is to get hit by an attack your current Digimon can use. Additionally, you can also train their Brain stat with the Classroom, but that takes much longer and is less likely to happen.
One of the characters in Live A Live is a professional fighter who is determined to become the best in the world. He learns new techniques from his enemies by having them done to him.
The King of Fighters's Rugal Bernstein was apparently originally supposed to be able to do this, taking your attacks and turning them back upon you, but due to memory constraints, he was given Geese's Reppuu Ken and Krauser's Kaiser Wave, with the explanation that he had fought those men before. He also has his own version of Athena's Psycho Reflector, called Dark Barrier.
Kingdom Hearts II has that and the Drive Forms. Each form's fighting style is a reflection of the person from whom the form is derived:
Valor Form: Reflects Goofy's all-physical, up-close-and-personal fighting style
Master Form: Reflects Mickey's fighting style, using physical, magical, up-close, and distance attacks
Final Form: Reflects Roxas's fighting style, using lightning-fast attacks and both Keyblade and Nobody-style attacks
The Final Mix-exclusive Limit Form reflects how Sora fought in the first game, using some of the special moves that were usable in Kingdom Hearts 1, but not the original sequel.
Kingdom Hearts: Birth by Sleep has D-Links, which gives you a new command deck, finishers and passive abilities based on friends you meet during the game. Some, such as the ones based on Terra, Aqua, Ven, Mickey, Donald, Goofy and Zack were fairly reflective of their fighting styles, while ones based on Cinderella and Snow White, for example, were mostly thematic.
Clive Barker's Jericho sees The Firstborn use this when you fight it. It attacks by launching bolts of lightning at members of the team. When it lands a hit, it temporarily absorbs the powers of the victim, and the only way to damage it is to possess that team member and use their powers against The Firstborn. Makes for some awesome animations; most notably the aerial duel between The Firstborn's version of Ababinili and Delgado's and the beatdown handed to it by Church.
DMC3 employs a different variation, where defeated bosses have their souls turned into Devil Arms for Dante's use (Cerberus becomes a three-handled ice nunchaku, Nevan becomes a literally-electric guitar, etc.). He also gains the Quicksilver style to slow time down after being on the receiving end of time-stops from Geryon.
SaGa Frontier has a single instance of this in Alkaiser's story. In the fight against the Affably Evil robot version of himself, upon using the move Dark Phoenix, Alkaiser can use his own special move in response to seeing his in order to learn his ultimate ability, Re-Al-Phoenix.
Also Monsters, and Riki in particular, are based entirely around absorbing enemies and mix-and-matching their moves.
Mystics can seal monsters into their "mystic" weapons to learn moves as well gain a stat bonus.
The Breath of Fire series. In many of the titles, you can copy specific enemy abilities by simply witnessing them (in III it was a Examine command, and in IV you merely had to be Defending.).
Marisa Kirisame of Touhou has a variation on this, where she actually steals the spells or spellbooks of others. She has only a small few spells that are actually of her own design (and she rarely even uses them), even her own signature move was stolen from Yuuka (although, since the Master Spark seems to be an innate ability, it seems to be a more 'pure' example of this trope than spellbook-based Non-Directional Laser). Since spellbooks are specifically stated in canon to be illegible to anyone who is not at least as great a mage as the one who wrote the spellbook when they wrote it, the fact that the "mere" human Marisa can swipe powers from Patchouli, and even Yuuka, implies she has at least in some way earned her powers.
It is worth noting that Marisa's versions of the spells she copies are often weaker than the originals in some way. Her Master Spark is smaller than Yuuka's, her Non-Directional Laser requires more power to cast, and her Orreries Sun cannot be thrown. She makes up for this by developing new spells which use the copied one as a base, often turning them Up to Eleven (Final Spark/Final Master Spark/Blazing Star, Starlight Typhoon and Orreries Solar System respectively).
Humorously, in the Windows era Marisa is something of a Composite Character of her prior incarnation and various other PC-98 characters. She has copied her appearance and personality.
One of the official supplementals, "Grimoire of Marisa", uses this as its framing device. Marisa compiles a list of other people's spell cards, and ranks them according to how useful it would be to copy them (rejecting many based on her inability to copy the Required Secondary Powers, her doubts on her ability to control them, or... well... because they're not worth the trouble to steal).
Also from Touhou, Satori Komeiji of Subterranean Animism is able to copy your partner's spellcards with her mind-reading powers, making her stage a Nostalgia Level.
And in the gaiden game Impossible Spell Card, Seija Kijin exemplifies her nature as a Commander Contrarian by, during the first time she's a playable character, adamantly refusing to use her own actual powers in favor of using cheat items that allow her to replicate the gimmicks of numerous other characters.
Shin Megami Tensei: Devil Survivor has a system in which your characters can "crack" the skills of demons or other demon tamers, gaining the skill to equip for later or immediate use. Devil Survivor 2 increases the odds of getting a skill cracked with Joint Cracks.
Shin Megami Tensei IV has Demon Whispers. Once a demon has reached a level where it has unlocked all the moves it's gonna learn naturally, they initiate the event, in which they offer to teach you any of their abilities, barring some special moves and passives.
The Maw features the title character as an Extreme Omnivore. When it eats certain unique enemies, it can also gain their powers, such as fire, flight, or shooting laser beams from its multiple eyes.
Marivel of Wild ARMs 2 and Emma of the Wild ARMs 1 remake build up their skill set by stealing/downloading the abilities of the monsters they meet.
One of the most popular skill builds of Ragnarok Online's rogues (and stalkers) involves using their Intimidate ability to copy the most recent skill they've been hit with, from player or monster alike. Only stalkers have an ability to preserve the copied skill, however.
The Izuna games have a rare talisman that allows you to capture any enemy within it, as long as it's at a lower leven than the talisman. You can then read the talisman to summon them to your side, rip it to unleash an attack depending on the monster caught, or stick it to a weapon to add various effects to your physical attacks.
Guild Wars has the mesmer profession which has access to a selection of "steal" skills, which enable them to use a random skill from the enemy's bar for 20 seconds.
Defeating anyone in Blood Storm would give the player a designated move.
Spellsteal in World of Warcraft pretty much works like this, although only for abilities that create a magic effect aka buff on the caster, which the spell transfers to the mage as the name implies.
A more straight example with the Death Knight "Dark Simulacrum" ability, which, when cast on a target, allows the death knight to instantly cast the next of the target's spells that uses mana without losing any effectiveness (For one cast, at least).
In War Craft III, the same spell could also be used to steal summoned creatures from the enemy.
The sneak preview at the upcoming expansion's talents gives us the Druid's Symbiosis ability which will copy one spell from the targeted player. That player will receive a druid spell.
Each time you defeat one of the five evil pigs in Tomba! 2, you get a special robe that will let you use the elemental magic of the evil pig that you beat.
A darker version in Myth. As part of the Vicious Cycle, the Leveler, a dark god that destroys the world every thousand years, reincarnates the last hero who defeated him and turns them into the next Leveler.
Viewtiful Joe has Joker, a recurring King MookMini-Boss. In your first encounter with him, his combat skills are limited to the standard Bianky punches and kicks, but in subsequent encounters, he adds additional abilities used by other Mooks to his repertoire until your last encounter with him where he has learned most of them.
In the roguelikeBrogue, your Monster Allies can study (or consume) the corpses of monsters to gain their abilities: things like teleportation, being immune to fire, or flying. Yes, you can have permanently flying Tentacle Horrors following you around.
In DotA and DotA 2 there's Rubick, a playable hero with the title of Grand Magus, who can use his ultimate ability, Spell Steal, to copy almost any previously cast active ability from a nearby enemy hero for 3-5 minutes into his own set of abilities.
The Big BadDowny Reed in Duel Savior Destiny has the ability to absorb the powers and abilities of others, either through simply copying what they do or flat out absorbing them and killing them in the process.
You Grabber in Grid Warrior does this, but can only be used on weakened foes. Depending on the enemy's strength, you'll get up to three upgrade levels per weapon. The final upgrade can only be obtained by finishing off a weakened Boss with the Grabber.
In StarCraft the Primal Zerg absorbed genetic material or "essence" from the creatures they ate. A number of missions in Starcraft II Heart Of The Swarm involve incorporating new abilities into Zerg strains by killing and consuming animals.
Grimoire Stones in Etrian Odyssey Untold: The Millenium Girl can contain enemies' skills along with regular class skills.
In Dust: An Elysian Tail, your projectile-casting sidekick Fidget somehow learns how to shoot fireballs and lightning, respectively, after defeating the first two bosses. Combining them with your character's Dust Storm ability replicates the bosses' special attacks and makes you a true force to be reckoned with.
Shovel Knight contains a downplayed example, as most of the relics you can find within stages give you a weapon that emulated a part of the boss's attack pattern or an attack based on one of that stage's themed enemies.
The magical Crystal of Absorb from Unforgotten Realms gives one the power to take on the powers of a dead opponent. Rob instantly equates this to being "like Mega Man".
Red Mage of 8-Bit Theater can copy the last action he's seen, though only once. This is because his class was changed to Mime — a real class from the Final Fantasy series — but the author uses it for occasional references to Marvel's Taskmaster as well.
Also on 8-Bit Theater, Black Mage has the blue magic ability to learn attacks from others. Naturally, since he's the official Butt Monkey, he has the most inconvenient form of blue magic - he needs to get hit with an attack to learn it. One of the only three that he's learned actually keeps him as the target when he uses it.
Black Mage: When Sarda casts a spell to hurt you and you learn that spell, you learn to cast a spell that hurts you.
That happens twice, actually. Sarda's simple "rewrite reality acording to my will" spell rewrites reality according to Sarda's will.
Chocolate Milkmaid can absorb the powers of anyone she "takes in a part of". Bodily fluids count. All of them. But it only works when she's in Milkmaid mode.
Max from Asperchu can copy an enemy's abilities by swallowing them, due to being an Expy of Kirby.
Chaka, at the Super Hero School Whateley Academy in the Whateley Universe, has the ability to manipulate Ki and see it flow. When attacked by a ninja using a secret "paralyzing nerve strike", she watches him as he tries it on her. She blocks it, so he never finished the move. She promptly uses it on him before he can react, having learned it in less time than it takes to execute the complete move once.
The second time she meets that ninja, she witnesses him finish off another student beforehand with a Kame Hame Hadoken. When she fights him shortly after, she takes him out in a Quick Draw battle, using the blast back against him. She names it the "Chaka Chaka Bang Bang". Chaka stealing Daikon's moves seems like it's one instance away from being a Running Gag.
Also, 'mimic' abilities are one possible subcategory of mutant powers in that setting, though the copy isn't usually quite as good as the original. For example, Sahar has the ability to 'collect' other people's specific psychic knacks, and Counterpoint is a school bully who deliberately tries to goad other students into fighting him so he can copy their powers and beat them up even worse next time.
Possibly the new canonical example of the Mimic trait is Mimeo, who can copy the powers of multiple mutants at once at full strength simply by fighting them for a while and as an experienced career supervillain also generally has the skill to use the powers effectively once he has them (it helps that he's studied at Whateley himself back in the day when the school was still new). His one limit in this regard is that he can only hold the copied powers for up to four hours — consequently, his usual modus operandi appears to be to show up when he needs cash, tussle with a team of superheroes for a while, and then go off to hit a bank or otherwise commit his real crime while the "charge" lasts. And now he's fought Team Kimba and discovered what those kids can do...
Jasmine, from Trinton Chronicles, has the ability to copy all the other superpowers she is with-in range of. This being said her power is kind of useless against others with the same or similar ability. Jasmine has copied so many powers at once she seems to have no limit, she also tends to use them like 'toys' in some cases including copying the power of a transmuter to transform junk into gemstones and the power of mentally interfacing with technologies in order to turn on her apartment's electronics and dial the phone without ever touching it.
Taltos does this also only using his own power of Nemesis against several of the heroes in Hallow's Eve
In the crossover of Smash Bros Lawl, The Irate Gamer has this as a signature move, but with the twist that the powers he copies are very underpowered versions of the originals.
In Worm, there have been two characters seen with the ability to use the powers of other parahumans:
Glaistig Uaine, who absorbs dead parahumans, and can manifest a ghostly version of them that retains their powers.
Grue after his second trigger event, who can borrow the powers of anyone he catches in his darkness.
Tohu, Endbringer number six, can take the powers of any three capes, including ones that aren't currently on the battlefield. Given the number of synergizing powers in Worm, this is, as normal for the Endbringers, a Story Breaker Power.
Victor can steal proficiency from others by proximity, but only mundane abilities. However, he retains these skills, while the victim of the skill drain can recover part of the lost proficiency with time.
The Butchers gain a weakened version of the powers anyone who ever killed the Butcher had
Ben 10 later gained the ability to turn into any alien the Omnitrix got a DNA sample from in the third season, allowing him to transform himself into slightly-altered versions of a small handful of that season's Aliens and Monsters. To date, each of these three forms has been used exactly once, and the plot point seems to have been dropped, although it was picked up again in Ultimate Alien.
Ben's nemesis Kevin 11 started out with a similar ability, which he used to copy Ben's powers. After getting Ben's powers, his original ability was never seen again (except in an "alternate future" episode where he had used his ability absorption to steal traits of thousands of aliens, basically becoming a cross between Naraku and Sylar).
Later seasons have Kevin as a sidekick, having found a way to return to human form. His power is the absorption ability he originally possessed, although he only uses it to absorb matter instead of copying powers. It's explained in a much later season that his power copying is a Dangerous Forbidden Technique that turns members of his species evil and insane, which is why he was a sociopathic villain in the original series.
The Big Bad of MIB: The Series was Alpha, the human founder of MIB, who merged with an alien artifact that allowed him to absorb the body-parts of aliens into his own, allowing him to cheat death and also resulting in him turning into a rather grotesque patchwork lifeform (with the abilities and attributes of all his alien body parts).
Blackarachnia from Transformers Animated can drain powers with a touch, similar to Rogue (except she can turn it off). Back when she was Elita-1, she could copy without draining. Lockdown, similar to Alpha, steals body parts from other Transformers, and is essentially a Frankenstein's monster-like assembly of parts.
Twilight Sparkle of My Little Pony: Friendship Is Magic is capable of this. While most unicorns are capable of a little magic that helps them in their special talent, Twilight's talent IS magic so this allows her to easily copy spells used by other unicorns. She only need be exposed to it a couple times at most in order to pull off a fairly accurate facsimile.
The Shushu Anathar from Wakfu can copy the magic of anyone he touches. He can even those use their magic better than they can. When he copied Yugo's power he could create dozens of portals at once while Yugo is only able to create one pair of portals at a time.
Mega Man's methods in the show differed from the games: Defeating a Robot Master wasn't needed as Mega Man only need to grab the Robot Master to absorb the power. His armor's colors also stay the same familiar blue, and the Robot Master's weaknesses to the weapons are never brought up. He could seemingly only keep the power for a few minutes/shots, but he stole a Robot Master's power at least Once an Episode, usually with stock footage.
Mega Man's powerful time-traveling ally, Mega Man X, gets in on the act and takes this up to eleven during his only appearance in the cartoon. X shows that not only can he copy enemy weaponry from Mega Man's time (Snake Man's Search Snake, to be precise), but like in the Mega Man X video games, X can super-charge them. He powers-up the Search Snake and sends it to speedily devour Wily's gigantic super laser.
When it returned, Amazo was basically a God. Lex Luthor later wanted to make himself an android body to gain Godhood using the same blueprints as Amazo.
On The Simpsons, Comic Book Guy invents a superhero named Everyman, who is able to get the superpowers of any superhero just by touching a comic book. "His superpower is every superpower!"
The nudibranch, a type of mollusk sometimes called the "sea slug," feeds on jellyfish and other stinging sea creatures. It is capable of taking the "stinging" cells from the creatures it eats and incorporating them into its own body, sometimes becoming more deadly than the animals it preys on.
Photuris fireflies first learn and mimic the blinking pattern of a different genus' females, then devour the attracted males. They can then absorb the devoured male's defensive toxins and use them for themselves.
It was believed by some cannibalistic tribes that eating a dead person's brain allowed you to learn their life's experiences. This unfortunately meant you could also gain the exact same disease they probably died from.
The most common version of this was when warriors would eat the brains or hearts of vanquished enemies to gain their strength or energy.
Eating brain matter was the main way the disease kuru spread between people. It's thought to be extinct now. Just as well since it's deadly and dying from it is messy at best.
The Belgian beer industry is home to several strange and exotic beers but is fully capable of mass-producing less bizarre styles that originated abroad, like porters, stouts, etc.
Strange and exotic to non-Belgians, perhaps, but not to them.
The concept of sympathetic magic can work like this: the Bimana of Africa decorate their hunting shirts with mouse skins for speed, animal claws and teeth for ferocity, and so on.
Rabies. A deadly example but the trope still works. Rabies does not just spread via bites but also from eating something with rabies. Without treatment, this can be fatal but before death hits, "Rabid" strength, speed, ferocity and foaming at the mouth can be enjoyed.
Bruce Lee's Jeet Kune Do works on the idea of "absorbing what is useful". Lee's personal style combined his best moves from Wing Chun, Tae Kwon Do, western boxing, fencing, and escrima. Anyone adhering to this concept of martial philosophy can build their own style by learning from different ones.
Many drugs are like this as certain (sometimes unnatural) chemicals in them mimics neurotransmitters. This can be beneficial if it gives your brain something it is lacking. It can be harmful if it causes you to get addicted and makes the real neurotransmitters less effective. It can be worse if you get too much of something, because then you body and mind freaks out and you can die from an over dose.
The mimic octopus doesn't gain "powers" per-say, but it can use it's transforming ability to scare off or discourage certain predators and it can change color to camouflage it self.
Viruses, though they aren't technically living (sort of, the other wiki uses this quote: "organisms at the edge of life"), invades a cell. If two viruses of the same strain attacks a cell, their DNA or RNA can break apart and recombine into a new shape. Also, if different strains of a virus have an "offspring", they can become more deadly AND spread across species. The 2009 flu pandemic, i.e., had a mix of swine, avian, and human influenza genetic sequences.
There a few animals that evolved to look like more dangerous animals, i.e., a non-venomous that looks similar to venomous one. They are other species that are more rare in that they mimic a more dangerous species behavior. There's at least one snake that mimics cobras despite the fact that its species has never seen cobras and there are no fossil records of its species meeting a snake that acts like a cobra (cobras' behavior really is one-of-a-kind, well, the only species that can back it up anyways...)