Adventure Time gave us "Five Short Graybles" (five loosely-connected shorts with the unifying theme of the five senses) and the ever-popular episode "Fionna and Cake", which Gender Flipped the entire cast.
The episodes that focus on Marceline and Simon Petrikov (before he became the Ice King).
"Star Trek": a Film Noir parody about Steve's rise to stardom after writing a book about Roger.
"Tear Jerker" and "For Black Eyes Only": full-episode James Bond parodies
"Blood Crieth Unto Heaven": a parody of the play "August: Osage County" with Patrick Stewart shown in live-action
The ass-kicking Christmas episodes they've done, such as "The Most Adequate Christmas Ever," "Rapture's Delight," "For Whom the Sleigh Bells Toll," "Seasons Beatings," and the temporarily Banned Episode "Minstrel Krampus".
"Hot Water": A Musical Episode featuring a killer hot tub. Was meant to be the final episode of the series as the writers were afraid that American Dad would be canceled, but when news hit that it was renewed, they put this as a season seven premiere and wrote off the deaths as non-canon.
"Lost in Space": A Day in the Limelight for Jeff and a completely straightforward (except for a few jokes) science-fiction story.
Animaniacs has an episode that mixes all of the skits' different characters together, making for such hilariously awkward combinations as "Mindy and the Brain," "Pesto and Runt", and "The Warner Bros and their sister Slappy."
They also did some sketches featuring none of the typical cast (e.g. "The Flame", a candle flame watching Thomas Jefferson write the Declaration of Independence) and/or more serious tones than the rest of the show (e.g. "Puttin' on the Blitz", a Rita & Runt segment set in World War II Poland).
Avatar: The Last Airbender had "Tales of Ba Sing Se," in which the episode was divided into a series of four-minute short stories, highlighting different characters and their individual adventures in the city of Ba Sing Se.
Not to mention the lampshade-happy "Ember Island Players", where the gang watches their adventures as performed by a (misinformed) theater troupe.
The Batman had "Artifacts", in which scientists a thousand years in the future discover the Batcave and interpret their findings while searching for a solution to a threat arising from an incident about twenty years after the show's main timeframe.
"The Man Who Killed Batman", where a nobody mob underling, "Sid the Squid", acts as the lookout on a job in which Batman is apparently killed. From Sid's perspective, Bats fell into a gas explosion, but everyone else thought Sid pushed him. He has to deal with mooks who want to prove their toughness by challenging him, a jealous Joker, an obsessed Commissioner Gordon, and a mob boss who thinks he's making up the whole story as part of some scheme. The producers admitted that this episode was an experiment to see if they could make a good episode with Batman appearing as little as possible (indeed, Batman only appears for about three minutes total in this episode). Arguably, they succeeded.
"I've Got Batman in My Basement", about two kids who take in an unconscious Batman and outwit the Penguin Home Alone-style. Unlike the others, this one was poorly received by fans.
"Showdown", where Ra's al Ghul tells the story of how his son Arkady Duvall was thwarted by Jonah Hex in 1883.
Aquaman's Outrageous Adventure! relegates Bats to a supporting role and instead focuses onAqua Man as he (begrudgingly) takes his family on vacation only to sneak away at any opportunity to help other heroes.
Chill of the Night! features a much darker plotline with many shout outs to TAS, a darker style of animation and an in depth look at Batman's origins (the show had previously stayed away from even showing Bruce without the mask, much less exploring his personal history).
The Knights of Tomorrow! shows a possible future for Batman and features the animation debut of Damian Wayne, the current Robin.
Bat-Mite Presents: Batman's Strangest Cases! was hosted by the 4th wall breaking Bat-Mite and showcased three bizarre interpretations of the Bat (all voiced by actors other than series star Diedrich Bader who voices a supporting part in one segment):
Bat Boy and Rubin, based off of the MAD parody of the same name, trying to solve a series of strange murders.
A segment based off of the manga version of Batman, done in a style similar to the Speed Racer cartoons. Features Batman and Robin facing off against "Lord Death Man" not long after he was introduced in the American comics by Grant Morrison.
Bold Beginnings! is told almost entirely in flashback as Aquaman, Green Arrow and Plastic Man exchange stories about the first time they teamed up with Batman while waiting to be rescued from Mr. Freeze. Almost a reverse of "Almost Got 'Im" from TAS.
Four Star Spectacular! is four shorts starring Adam Strange, The Flash, 'Mazing Man and The Creature Commandos with Batman only making cameo appearances.
In "Meet Polka Dots" on Blue's Clues, Blue opened the door to the Blue's Clues house and there were several segments in which time was spent with just Blue or Blue and her friends rather than Joe, while Joe attended to his stuffed duck Boris, who was having a nap. Perhaps most unusually, viewers got to watch as Blue placed her pawprint on each clue, something which had always been done firmly off-screen before.
Courage the Cowardly Dog has "Cowboy Courage", which takes a break fro the supernatural horror and instead has Courage dreaming about being a sheriff in the old west.
There is also the episode "Aqua Farmer", which is almost entirely about Eustace, in lieu of Courage as the protagonist. (In contrast, he is completely absent from "Cajun Granny Stew", although this could be chalked up to Early Installment Weirdness.)
The Danger Mouse episode "Once Upon A Timeslip" takes the cast to Medieval days to enact a Robin Hood story at the behest of the announcer and a quirky microphone. DM lampshades it early on:
DM: The announcer's voice is controlling the picture, dictating what we do!
Penfold: Crumbs, chief. What are we gonna do?
DM: Dunno...have a dafter adventure than usual, I suppose.
"Darkwing Doubloon" portrays Darkwing and his supporting cast as daring swashbucklers out to stop a dreaded band of pirates as played by the Fearsome Five.
"The Secret Origin of Darkwing Duck" features a framing device set in the far off future where an old janitor who may or may not be Darkwing's ghost telling two kids resembling Gosalyn and Honker his true origin, in which he was sent to Earth as a baby after his planet exploded.
"A Star is Scorned" takes place in the real world with Darkwing and the rest of the cast as actors and themselves at the same time with Darkwing trying to keep his role as star of the show.
There's also "Blackfoot and Slim", which is an episode presented as a nature documentry that observes Dexter (or Blackfoot as he is called in this episode) and his daily activities until he realises what's going on and tries to run away from them until he is shot with a tranquilizer dart.
"Framed", a completely normal Slice of Life story about Dexter's broken glasses starting a trend at school. It's very dialogue-heavy and introduces a bunch of characters that are never seen again.
Not technically true. Becky and Glenn do re-appear in a few other episodes, albeit Demoted to Extra.
Duckman had "American Dicks", presented as a detective show program on its 100th episode with Duckman as the guest detective, complete with Shaky Cam. Ultimately, it was Cornfed who solved the case.
The three-part Star Wars parody episodes: "Blue Harvest", "Something Something Something Dark Side", and "It's a Trap!", because they follow one continuous story, or parody if you will, throughout the entire episode.
"Brian and Stewie": A Bottle Episode based on the All in the Family episode "Archie in the Cellar" where Brian and Stewie (meaning that it's Seth MacFarlane talking to himself for 22 minutes) are locked in a bank vault for an entire weekend and play off each other.
The season nine premiere "And Then There Were Fewer" is a feature-length episode (and the first episode to be shown in high-definition) presented as an almost straight Agatha Christie style murder mystery, in which several recurring characters are bumped off permanently.
The two "Treehouse of Horror"-style What If? episodes ("Anthology of Interest"), meant to be a yearly thing.
"The Futurama Holiday Spectacular" is made of three unrelated vignettes centered on the three 31st century holidays - Xmas, Robanukah, and Kwanzaa - with a Green Aesop thrown in.
"Naturama", which is done in the style of a wildlife documentary with the cast portrayed as talking animals.
"Reincarnation"'s asks "What if Futurama was an early 1930s black-and-white cartoon, or a 1980s video game, or a Japanese cartoon from the 1970s?"
"Saturday Morning Fun Pit" had a framing device with Richard Nixon's head and the headless body of Spiro Agnew trying to watch Saturday morning cartoons and having to deal with angry Moral Guardian protesters. The cartoon parodies shown were based on American cartoons (Scooby Doo, Strawberry Shortcake, and G.I. Joe), and the episode made fun of just how irritating the Saturday morning TV experience is once you take off the nostalgia goggles: corny jokes, crummy writing, mediocre animation, media watchdog groups complaining about children's cartoons being too violent, and a lot of shows being little more than 30-minute commercials for toys and cereal.
Any Bonds Today is only one and a half minute long and therefore the shortest officially released Looney Tunes cartoon ever. It also features no plot, except Bugs, Porky and Elmer Fudd singing and dancing to a song to make the audience buy war bonds to support the war effort. In terms of propaganda cartoons it's the most blatant direct example.
Chuck Jones was known for experimenting with animation in many cartoons: The Dover Boys (1942) features stylized and limited animation almost a decade before it became fashionable. Duck Amuck (1953) is unusual in the sense that it stars Daffy Duck throughout most of the entire cartoon, who is constantly changed, muted, erased,... to the point that he is actually seldom in this cartoon as his own self. "Now Hear This" is a rather unique and experimental cartoon as well.
One episode of Martha Speaks "Return of the Bookbots: the Case of the Missing Words" was an extended episode of the Bookbots, a potential TV series Martha had pitched back in "The Martha Show pt. 2". None of the regular characters appear outside of their Bookbot forms well, at least not until the end, anyway.
Unlike most examples, in Oggy and the Cockroaches, this applies, not to an episode, but to an entire season. Taking a cue from The Movie, the fifth season will showcase Oggy and the Cockroaches' conflict throughout history. It is currently unknown whether or not episodes set in modern times will happen.
"The Wizard Of Odd" focused not on Phineas and Ferb's project of the day, but on Candace as the center of a Wizard of Oz-type plot.
The Pink Panther short "Pink-Outs" is different from the others, as it is just 12 short vignettes about the Panther engaging in various gags. These would eventually be used as bumpers on The Pink Panther Show.
The Sealab 2021 episode "7211" is basically free of humor, because the Sealab 2020 episode on which is is based (in which the crew works fast to save another submarine from nuclear meltdown) is left mostly unchanged, merely trimmed for time and re-dubbed in the 2021 voices. In the context of a series filled with non-kid-appropriate jokes and a crew that never gets its act together, the effect is Anti-Humor.
Every episode of Sheep in the Big City ends with a rant from the Ranting Swede, except one that didn't. "Due to a scheduling conflict, we now present The Ranting Norwegian..."
Sid the Science Kid is normally a very Strictly Formula type of show, in which Sid would always wonder about something in the morning, the go to school and learn about it with his teacher and friends. However, there were a couple of breaks from this. The "No School Sing-Along Special" featured Sid and his friends on a day off when they decided to do a sort-of concert thing. There was also "Sid's Backyard Campout," in which Sid, his friend Gerald and his father, Mort, had a campout in Sid's backyard and explored nature and such.
Silly Symphonies were considered something completely different at the time. Walt had just struck gold with the succes of Mickey Mouse and could have easily made nothing but shorts starring Mickey, but he decided to start another animated series which told stories where the music was dominant and fairy tales and nursery rhymes were adapted. These series had no recurring characters either.
The clip show episodes "So It's Come to This: A Simpsons Clip Show," "Another Simpsons Clip Show," "The Simpsons 138th Episode Spectactular," "All Singing, All Dancing," and "Gump Roast."
The non-Halloween Three Shorts episodes ("Simpson Bible Stories", "Simpson Tall Tales", "Tales From The Public Domain", "Margical History Tour", "Simpsons Christmas Stories", "The Wettest Stories Ever Told", "Revenge Is A Dish Best Served Three Times", "Four Great Women and a Manicure", and "The Fight Before Christmas").
"Twenty-Two Short Films About Springfield": An aborted Back Door Pilot showing what life is life for everyone else in Springfield besides The Simpson family.
"Behind the Laughter": A parody of Behind The Music and E! True Hollywood Story showing The Simpsons as Animated Actors and the ups and downs of their fame.
"Springfield Up": A documentary visiting the adult population of Springfield who were interviewed as kids.
"The Springfield Files": Leonard Nimoy tells the story of how Homer encountered an alien.
"The Simpsons Spinoff Showcase" and "The Simpsons 138th Episode Spectacular": Troy McClure hosting, and a lot of fourth-wall-breaking jokes about the show. "The Simpsons 138th Episode Spectacular" is also the only Clip Show episode where it doesn't involve the family sitting around and remembering past episodes.
The flashback episodes ("The Way We Was"note how Homer and Marge met in high school, "I Married Marge"note how Homer got Marge pregnant, married her, and applied as a nuclear plant worker, "Lisa's First Word"note how Marge and Homer moved into their current house, how Marge got pregnant with Lisa, how Homer met Flanders and began borrowing stuff from him, and how Bart was jealous over Lisa, "And Maggie Makes Three"note how Homer quit his job at the nuclear plant, only to re-apply after Marge becomes pregnant with Maggie, "The Way We Wasn't"note how Homer and Marge unknowingly met as kids at a summer camp, "Homer's Barbershop Quartet"note a Beatles parody episode centered on Homer and his friends becoming music stars in the mid-1980s, and "That 90s Show"note how Marge and Homer broke up in the 90s before they had Bart).
"A Million Little Fibers" features none of the main cast. It does, however, feature a talking towel, Oprah Winfrey, a talking anus and a talking vagina.
"Goth Kids 3: Dawn of the Posers" was entirely focused on the Goth kids, with none of the main cast again. Since the show missed the deadline for the first time ever, a rerun of another episode was aired, and when "Goth Kids" aired many fans felt rather underwhelmed.
The Spongebob Squarepants episode "Krusty Krab Training Video" plays out the style of an introductory video being played for a newly-hired Krusty Krab employee.
Star Wars Rebels: "Twin Suns" is the first episode of the show where the focus of the plot is not on any member of the Ghost crew, and they're all in supporting roles. What's this episode about, by the way? The final showdown between Darth Maul and Obi-Wan Kenobi.
In Teen Titans the series finale, "Things Change", kept the fight with the Monster of the Week to a small B-plot (and never did get around to resolving it), while most of the episode focused on Beast Boy trying to reconnect with a girl who may or may not be Terra. While the series always dealt with emotional issues, usually a layer of superhero metaphor was applied. Here it was simply, bittersweetly Slice of Life.
The final season had a couple of these. Another episode dealt entirely with Raven babysitting a trio of superpowered toddlers, and another was about the HIVE Five trying to capture Kid Flash, and Jinx's High-Heel–Face Turn.
Transformers Armada had an episode that focused entirely on the Decepticons. Rad, the usual narrator, was replaced by Sideways, and the Autobots got barely any screen time, with the kids entirely absent.