Superhero Episode

Fry: When you were a kid, what was your greatest fantasy?
Leela: To have parents.

Fry: Whatever. The correct answer is: To be a superhero! We have superpowers, and we're Americans. Now's our chance!

—Futurama, "Less Than Hero"

An episode of a non-superhero show in which the characters temporarily gain superpowers and/or the urge to dress up and fight crime.

The characters in question may normally be non-powered, or they may have already had extraordinary abilities that they've never before referred to as superpowers.

Depending on the nature of the show's universe, this may occur "for real," as a dream or fantasy, or completely out-of-continuity, but in any case it involves familiar characters wearing capes, kicking ass, and going by (possibly parodic) Something Person names.

May include a stock parody fight scene referencing the '60s Batman series.

A subtrope of Something Completely Different.

Examples of Superhero Episode include:

Anime and Manga

Dragonball Z's Saiyaman.
Episode 14 of Seikon no Qwaser, with Lily Mask.
One Piece has a few shorts and one numbered anime episode revolving around "Chopperman", a superhero version of Chopper completely out of continuity, where Chopper plays the eponymous hero, Usopp the Big Bad, Zoro, Sanji, and Robin form the Quirky Miniboss Squad, Nami is Chopper's secretary, and Luffy is a giant sombrero robot.

Comic Books

A relatively short lived Alternate Continuity of Archie features the characters as superheroes.
Strangers in Paradise had an out-of-continuity issue in which Francine and Katchoo were superheroes and Freddie a bumbling villain who accidentally destroys the world.
The Super Sidekick Sleepover Slaughter arc of Hack Slash.

Live Action TV

The Suite Life of Zack and Cody had one as part of a Disney "make a wish" event.
Sabrina the Teenage Witch had Harvey become Ultra-Teen or somesuch as result of a spell, though this wasn't the focus of the episode.
Charmed did an episode where a boy can bring his superhero drawings to life.
Soap opera Guiding Light of all things. There was even a corresponding comic book tie-in!
Superpowers broke out on an episode of Gilligan's Island thanks to a shipment of radioactive vegetables.
Stargate SG-1 episode "Upgrades." SG-1 gets superpowers after being guinea pigs for a Tok'ra technology. Their newfound powers cause them to completely lack good judgment, including getting in a bar fight.
Dexter had an episode in which Dexter imagined his life as a super hero (or super villain, he wasn't quite sure). As one would imagine, it was done in the same horrifyingly psychotic way as everything else in the show.
Sanctuary did it twice, in Hero and Hero 2, with a living suit which gives people superpowers but makes them unstable.
Warehouse 13 did it in the episode Mild Mannered; given the show's premise it was easy for an artifact to give someone super-powers.
Bewitched had the episode Super Arthur in which Uncle Arthur turns into Superman after Dr. Bombay gives him a pill.
The Monkees turn into "Monkeemen" in a few episodes. While they can fly, the rest of their powers consist of exchanging insults to bruise someone's ego.
While Lois and Clark was a superhero series, Lois Lane was most definitely not a superhero—except for one episode where she got Superman's powers and became Ultra Woman.

Newspaper Comics

Calvin and Hobbes has Stupendous Man strips from time to time. Some of these are Imagine Spots, while others have Calvin actually putting on a costume.

Webcomics

The Snowsong (a.k.a. Supergreg) arc in Dominic Deegan. It was entirely in continuity, unfortunately, and is one of several candidates for the comic's Jump the Shark moment.

Western Animation

The Futurama episode "Less Than Hero," in which Fry and Leela gain Super Strength, Super Speed, and Not Quite Flight Wall Jump as side effects of a medicated muscle rub. They are soon joined by Bender (despite his being "more on the supply side of crime"), who could do cool things already.
- They did not have the power to command crustaceans, though (one of the potential side effects of Miracle Cream).
The Simpsons had Bart and Lisa (as Stretch Dude and Clobber Girl) attempting to foil the Villain Comic Book Guy (the Collector) during one of their A Day at the Bizarro Halloween episodes.
- In a canon episode, Homer becomes the vigilante Pie Man, who throws pies at people while wearing a pie-tin mask on his face. The episode was largely a parody of the Spider Man movies.
Family Guy had the family mutated by a crashed toxic waste truck. Most of their powers were "normal", but Meg's power was super amazing fingernail growth.
Fairly Oddparents' Big Superhero Wish episode turned everyone in the world into a superhero/villan.
- Fairly Oddparents loves superhero episodes. Timmy is, after all, Cleft the Chin Wonder. And the Masked Magician. And occasionally Crash Nebula. And his parents are Mighty Mom and Dyno Dad.
In the Bugs Bunny cartoon "Super-Rabbit", Bugs acquires carrots which, when eaten, temporarily give him the abilities of super strength and speed, flight and invulnerability. Later, the two villains obtain the carrots and gain super powers of their own.
- Followed up immediately by a Shout-Out to, in the eyes of just about all back then and still many today, some real-life superheroes... the US Marine Corps.
  - ...leading the USMC to officially induct Bugs as an honorary private in the Marine Corps. He was regularly promoted until "retiring" at the end of the war as a Master Sergeant.

Bugs: -Sees the villains in Super form- This looks like a job for a real superman! -Dives into a phone booth, and emerges in Marine Corps dress blues-

- Also from Looney Tunes, we have Daffy Duck as "Stuperduck".
An episode of Felix the Cat had this. Apparently, his "Magic Bag" wasn't super enough on its own to count as making him a superhero, but it could just give him tights, super-powers, and a big beefy physique.
In Kim Possible, Kim and Ron each have a Superhero Episode—in "Go Team Go", Kim temporarily gains Hego's Super Strength, and in "The Fearless Ferret", Ron temporarily becomes the Sidekick of the TV-Batman Expy hero (actually a delusional actor).
The Duck Dodgers episode "The Green Loontern", where Dodgers accidentally gets Green Lantern's uniform and ring from the dry cleaners.
Jackie Chan Adventures had an episode where Jade was inflicted with all the powers of the talismans. She then dons a cape and calls herself "T-Girl." She reprises this role in another episode although with fewer powers. In season five, Jade's friend Larry gets the power of the Earth demon and starts behaving like a superhero with disastrous results.
In the first-season Pac-Man episode "Super Ghosts", Mezmaron's formula gives the Ghost Monsters super strength and other powers. Afterward, Pac-Man and his wife eat special power pellets that enable them to fly.
Monster Buster Club has an episode ("Comic Book Heroes") that features an alien nerd using the Earth as a place to store his billions of comic books. Cathy gets a hold of one series, "Liberty Lass", and pronounces herself a superheroine, Cathy Lass. She doesn't gain any new powers, but that doesn't matter because she usually has whatever wacky alien power she needs for any given situation anyway.
Foster's Home for Imaginary Friends: "Challenge of the Super Friends"
SpongeBob SquarePants One episode has Spongebob, Patrick, Sandy and Squidward all gain superpowers to fight Mermaid Man's enemies. Amusingly, their powers come from wearing the ridiculous costumes.
Jimmy Two-Shoes featured an episode in the first season wherein the main characters became Power Squid and Spaghetti Beezy, though unlike the typical formula for this trope, the powers were incredibly unorthodox and season 2 would occasionally have the characters become their superhero forms again if the need arose.
Interestingly, The Powerpuff Girls had one of these despite the main characters already being superheroes- after reading comic books, the girls decide to copy their favorite heroines in order to become better heroes, with Blossom, Buttercup and Bubbles taking on new heroic identities as Liberty Belle (notably the actual name of a Legacy Heroine in the Justice Society of America), Spore (a Spawn Expy), and an Animesque cute rabbit character along the lines of Hello Kitty. It doesn't go well.
Played around with Irma in the 1987 TMNT episode "Super Irma".
- The 2003 series has several, as a Justice League type team actually exists in this world, and Michelangelo occasionally dips into his persona as "The Turtle Titan". Then there's the Alternate Reality Episode where he encounters the main cast in a world where they are ALL superheroes and Splinter is their archenemy.
The 1987 has several more Superhero Episodes: "Raphael Vs. The Volcano," "Night of the Dark Turtle," and "Super Hero For a Day."
In Garfield and Friends, Orson's Farm occasionally featured Orson becoming "Power Pig" In both "reality" and dream sequences. In reality, his only powers seemed to be having a stupid costume (in his debut episode, he was used to distract the villain (a fox) with his appearance, which caused the fox to fall down laughing); in dream sequences, his powers consisted of something along the lines of Superman's (super strength and flying). In dream sequences, which usually consisted of a jazzed up version of the problem of the episode being solved, he was used as an inspiration for what Orson did in "real life".
Phineas and Ferb: The Beak.
Ed, Edd 'n' Eddy: "Robbin' Ed" has Johnny 2x4 and Plank as "Captain Melon-head and Splinter the Wonder Wood", who steal back the money Eddy conned off the other kids. This drives Eddy to plot revenge as the super-villain "Professor Scam".
An episode of Men in Black The Series had an alien immigrant decide to become a superhero, so MIB HQ decides to make Agent J into a supervillain to stop him, and all the attention he's getting.
My Little Pony Friendship Is Magic has "The Mysterious Mare-Do-Well", an episode where Rainbow Dash goes on an ego trip after saving some ponies from accidents and being called a hero. Her is thunder stolen by the eponymous masked pony. Mare-Do-Well's costume is a homage to Darkwing Duck's, and some of her fans are seen carrying signs with a parody of the Batman: The Animated Series logo on them.
Jimmy Neutron had the N-Men episode, where in an homage to the Fantastic Four, the kids are on their way back to Earth in their space ship (won on a game show in a previous episode) and accidentally get superpowers while passing through the Van Allen belt, each powers coming from what they were doing at the point.
- Jimmy was drinking OJ (becoming orange and going into a Hulk-like state), Cindy was armwrestling (becoming Wonder Woman, essentially), Libby was putting on vanishing cream (gaining invisibility and force field abilities), Sheen was on a massaging chair (becoming the Flash... his bladder suffered) and Carl belching while drinking soda (gaining super sonic belching abilities).
DuckTales (1987) notably had a superhero character added into the second season in Gizmo Duck, but one episode entitled "The Masked Mallard" notably featured Scrooge becoming a masked vigilante.