Badass Transplant

The Badass Transplant is when a character's awesome abilities aren't innate, but equipped. The hallmark of a Badass Transplant is that the part transplanted is significantly more badass than the rest of him, or even the source of his abilities entirely, and not something he obtained naturally. It might be mechanical, magical, or even possessed. If the transplant is from a particularly powerful entity, it might result in a Deadly Upgrade, a Super-Powered Evil Side, or turn on its wielder. Frequent overlaps with Artificial Limbs and Arm Cannon.

Although organic parts almost always apply, weapon upgrades don't unless used previously by a proven badass. (For example, Mega Man X getting an upgrade isn't a Badass Transplant, but getting Zero's Z-Saber is; the saber itself is badass.)

Examples of Badass Transplant include:

Anime and Manga

Naruto: Kakashi has a transplanted Sharingan, and uses it better than the Uchiha that he got it from.
- Similarly, Danzo has transplanted Sharingan. Except there's twelve. And they're on his arm.
Ginko in Mushishi has an artificial eye souped up with injected mushi.
Tsubasa Reservoir Chronicle. Albeit, only one party was willing...
Allen Walker's arm, which just keeps getting more badass.
Clare's new arm in Claymore.
- Technically, any transplant Clare gets is a Badass Transplant, since she tends to get the new parts from badasses.
Scar in Fullmetal Alchemist has his brother's arm, tattooed with alchemical markings that allow him to destroy anything with a touch.
- Edward with automail arm and leg counts too.
- Don't forget Ling getting Greed.
In Tengen Toppa Gurren Lagann, Kittan used Gurren Lagann's drill to destroy the Death Spiral Machine.
Sesshomaru of Inuyasha goes through a bunch of replacement arms - including a dragon claw - before giving up on the idea. He only needs one, anyway.
Guts of Berserk replaces the left forearm that he lost during the Eclipse with a metal replacement that can smash opponents senseless and mount a repeating crossbow. As a bonus, it's got a built-in cannon.
Hiei from Yu Yu Hakusho and his Jagan eye.
In Witchblade manga a young man was rather shocked to discover that the new skin on his arm is transplanted from his girlfriend. Then that she's already regenerated it. Then that she's a half-demon. It has some interesting effects, including changes on the rest of his arm. But probably worse than it would be if this wasn't the hand with which he wields his magical demon-slaying sword. Oops.
Ai from Dragon Crisis.
The Kamen Rider Spirits version of Riderman. Prosthetic arm? Check. Rope Arm (to grab things at a distance - think Link's Hookshot)? Check. Power Arm (a giant claw)? Check. Drill Arm (go on - guess)? Check. Machine Gun Arm? Check. The ability to chain them together to break through some nigh-unbreakable armor? Score one for the (nearly-)normal guy.
Legato's Mind Manipulation ability in Trigun is because he has Vash's arm.
Niki in Urotsukidoji replaces his own penis with a demon's.

Comic Books

Lobster Random's claws.

Film

Used similarly by Spooner in I Robot to kill the Big Bad. Also served as both The Reveal and an I Am Not Left-Handed moment in an earlier fight with a robot.
Evil Dead's Ash and his trademark chainsaw.
Cherry Darling in Planet Terror who has a machine gun for a leg.

Literature

Deliberately invoked by Tsavong Lah in the New Jedi Order. His people, the Yuuzhan Vong use transplanted limbs all the time, but after Lah's foot gets torn off in a duel, he goes for something special. He kills a Vua'sa [a nearly unkillable predator from his homeworld] with his bare hands and tears off one of its feet to replace his own. His subordinates were... impressed, to say the least.
Max Barry's Machine Man has Dr. Charles Neumann, who begins with one, then a pair of powerful prosthetic legs that can not only leap tall buildings In a Single Bound, but also kick down re-inforced steel doors. His team also designs powerful arms that end up attached to the Security Guard, Carl.

Live-Action TV

Riderman from Kamen Rider V 3 is apparently a normal dude in a powered suit... with a prosthetic right arm from the elbow down. Said arm can change forms into a rope-based arm, a giant claw, and a drill - at least in the live-action versions of him. He's not nearly as powerful as V3 himself, but makes up for it in effort.

Tabletop Games

Among the artifact-level magic items available in Dungeons & Dragons are the Hand of Vecna and the Eye of Vecna, which were formerly body parts of a powerful lich. To use one of these items, a character must first remove the appropriate part from his or her own body. (Rumors of an artifact called "The Head of Vecna" should probably be disregarded as the invention of a group of sadistic players with a sick sense of humor.)
In Hunter: The Vigil, this is the Cheiron group's specialty. What do you want? Demonic eyes that can see into the very depths of a man's soul? Trivial. A personal defence swarm of angry magical insects embedded into your arm? Child's play. A rotten human hand that can shoot fire, and hypnotize those that look at it? Why the hell not?
Most games that allow cybernetics (Shadowrun comes to mind; even GURPS or Dungeons & Dragons in some settings) will have a player who believes in this, and acts on it.

Video Games

Possibly the earliest in the medium: Bionic Commando's arm.
Gene's God Hand in God Hand.
- Also, Gene's Evil Counterpart, Azel, who possess the other arm of the pair.
- Then Gene gets both of them as an Eleventh-Hour Superpower.
Fate/stay night has Shirou get Archer's arm in the Heaven's Feel scenario, which doubles as a Deadly Upgrade.
- Odd variation here: It's still his own arm, just a bit older
  - Which is the only reason the transplant was successful.
Planescape: Torment has eyes that act as equipment for the Nameless One. He equips them by ripping out his old one and sticking the new one in its socket.
Ragna's replacement arm in BlazBlue, formed from the corpse of an Eldritch Abomination.
- And his second transplant. Though not quite to the Eldritch Abomination levels of the Azure Grimore, he receives an arm made from whats left of Lambda 11's regeneration tank.
Shepard's extensive body modification courtesy of Cerberus in Mass Effect 2 make him/her much, much cooler than normal humans.
"Meet the Medic" shows us that the Ubercharge in Team Fortress 2 is initiated by implanting the heart of a Mega Baboon with an implant attached in place of the trooper's normal heart (which tends to explode when exposed to the implant's charge).
Deus Ex Human Revolution: Adam Jensen is much more Badass after having his arms replaced with bionic ones, among other things. Most of them weren't even needed to save his life and were installed for the sole purpose of making him the most badass cyborg created by Sarif Industries.

Web Original

Michael gets this twice in Ather City. Once, after he loses his right arm and gets a replacement made of brimstone, and then Nick builds him a mechanical version.
- To clarify, before he lost the arm, he was considered the best fighter in the city. After, it's hinted that he could beat anyone in the series if he wanted to. This is after almost every single character takes one or more levels in one thing or another.
Zokusho Comics: Rotting Johnny takes this to a whole new level. He used to be a hitman, but someone blew his hand off. Just as he was considering exiting life via a shotgun, a mage came to him with an offer of a new hand. The hand worked great. Until it started rotting. Then the rest of his body decided to follow the new hand's lead. On the bright side, he can now graft fresh parts on in place of the rotting ones.