"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.
Many, many times, including "Butters' Very Own Episode", devoted to side character Leopold "Butters" Stotch; several musical episodes; "It Hits The Fan", which set out to break the world record for the number of inclusions of the word "shit" (162 in total); "Good Times With Weapons", an anime parody; the third act of "I Should Have Never Gone Ziplining" which is done completely in live action with adult actors etc. One of the most notable is probably The Movie, which is both deliberately cruder than the TV series (it has an anti-censorship theme, as well as an "R" rating), and a full-fledged musical.
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 ("The Fight Before Christmas," "Four Great Women and a Manicure," "The Wettest Stories Ever Told," and "Simpsons Christmas Stories")
"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 met as kids at a summer camp, totally retconning the high school story, "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 similar to "Homer's Barbershop Quartet, only the continuity is really wrecked and the episode is not as beloved.
"Bart vs. Thanksgiving" and "Like Father, Like Clown" are some of the few Simpsons episodes that were more dramatic than comedic and have the feel of a less preachy Very Special Episode.
Futurama occasionally showed flashback-based, almost art-house style episodes.
They also did two "Treehouse of Horror"-style episodes ("Anthology of Interest"), meant to be a yearly thing.
"Naturama", which is done in the style of a wildlife documentary with the cast portrayed as talking animals.
"Reincarnation" and "Saturday Morning Fun Pit" depict the characters and their adventures in the styles of other types of animation, though there are differences. "Reincarnation"'s doesn't have a framing device and doesn't make fun of the animated genres it shows. It's more like "What if Futurama was an early 1930s black-and-white cartoon, or a 1980s video game, or a Japanese cartoon from the 1970s?" Meanwhile, "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.
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.
"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.
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.
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.
The three-part Star Wars parody episodes: "Blue Harvest", "Something Something Something Dark Side", and "It's a Trap!".
"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 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 a jealous Joker, an obsessed Commissioner Gordon, and a few rival mob bosses who can't believe his story. 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.
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 for 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 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.
"The Wizard Of Odd" didn't feature any kind of project from Phineas and Ferb at all, instead focusing on Candace as the center of a The Wizard of Oz-type plot.
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.
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.
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).
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.
Animaniacs has an episode that mixes all of the skits' different characters together, making for such hilariously awkward combinations such 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).
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..."
"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.