A type of satire performed by adding humorousnote ("humor" ranging anywhere from tongue-in-cheek to outright vitriol) running commentary on top of another work.
The term originates from the fanfiction scene. There, a few lines of the original source material is quoted verbatim, with snarky jokes and comments posted inbetween quotes, like so:
> [Text straight from the fanfiction] MSTer: Witty comment!
The end result is similar to Mystery Science Theater 3000 (hence the term: MST). The practice originally used the cast of that show verbatim (extremely dedicated ones even including The Mads and the Invention Exchange), but eventually alternate casts (typically the person doing the MST, or sometimes characters from the material the fanfiction was based on) were used instead.
In some fandoms, this process is called "sporking". Outside of fanfiction, it's more commonly referred to "riffing" (not to be confused with the musical term). Text riffs are generally similar to fanfiction MSTs; audio or video riffs generally have the original audio present, with the snarky commentary running over it. This trope encompasses both forms.
Compare Alternate DVD Commentary, The Abridged Series and The Heckler.
Rifftrax: Mostly blockbuster movies and public domain shorts.
Retsupurae: Let's Play videos. There are several groups:
slowbeef and Diabetus are the original pair, and run the titular Retsupurae YouTube account. The pair eventually branched out to bad Flash games (typically of the horror variety) and longplays of bad or bizarre video games.
Chip Cheezum and General Ironicus occasionally do some Retsupuraes on an alternate channel of theirs.
The Vampoife Dood Who Lifed starts out with the characters MSTing an awful fanfiction created by the author his/herself, then morphs into an actual story... though there's still quite a bit of snarking.
The many Harry Potter fanfics in which the Marauders (or any other combination of characters!) somehow get hold of one or more of the canon books and read through it, making snarky comments in a Script Fic format.
There's also a fic in which the Marauders get hold of My Immortal (found here).* The Department Of Mysteries is a newborn sporking community for Harry Potter badfics, started by Snowfox and the newbie Gamerex27, both Tropers on this very website.
The LiveJournal community house_mst is dedicated to MSTing the very worst of House fanfiction, using characters from House to comment on it rather than the original MST3K characters.
A MSTing For All Seasons is a website collecting the MSTing works of three separate authors, targeting everything from the infamous "Oscar" stories to a collection of UFO conspiracy theory postings. They may be some of the best examples out there.
Specifically, prolific MSTer Megane 6.7, who's been riffing on fics for nearly a decade (including several MSTs done well after the zenith of the MiSTing community's output). Ever wonder who to thank/beat for finding Oscar's fanfics? Go find Megane; he was the first person to riff "Artemis's Lover".
The MSTing Mine is an effort to archive all MSTs posted to the MST3K Usenet groups.
Fanfiction.net decided to ban fic MSTings at one point, because they were "disrespectful" to authors.
Apparently, they have changed their policy or stopped caring since then, since this story is still going strong.
Topless Robot's Fan Fiction Friday feature does this for fanfic. It moved to io9 with creator Rob Bricken, but similarly to the Fanfiction.net example did not last long due in part to the "disrespect" issue.
A Random MST is a series that targets particularly bad fanfiction and other stories. Some exceptions, such as stories that aren't particularly bad, are made, such as the two MSTs of both Pokémon: Attack of Mewtwo and its sequel.
Retsuraed is described as 'halfway between an MST and an Abridged Series'. Besides the usual snarkyness, the people reading the fic often like to add additional 'scenes' to it that poke fun at the blatant plot holes and Fridge Logic in the fics.
The LiveJournal community badfic_quotes sporks multifandom fanfic and "professional" stories, with a particular love of poorly concieved summaries from The Pit (fanfiction.net), while weepingcock is the smutfic sporking equivalent, hunting down the worst descriptions of sex and body parts that will having you screaming for Brain Bleach. Particularly of note: the dark, dark world of Transformational Porn Fic.
The blog Library of the Damned riffs on fanfics from a variety of fandoms, ranging from video games to children's TV shows.
The Daily Insanity, made by Raetekusu. As of now, he only has Chapters 1-11 of My Immortal and a MST of an article comparing Stephenie Meyer to J.K. Rowling, but under planned MSTs, he has My Inner Life, a Sue-fic from Kingdom Hearts, and a couple of more recent fics he's found.
Das Sporking has sporkings for sixty-four fandoms and counting, including both fanfiction and published works. It initially had a much smaller range of fandoms to be sporked when the only sporkers were Das Mervin and Mrs. Hyde (who of course could only spork for things they were familiar with or interested in), however the community eventually opened up to allow more sporkers, all of whom brought their own fandoms to the table.
Twitter account fanfiction_txt finds particularly bad lines from fanfiction of all media, and presents them without comment. Tumblr account fanfictionimg then periodically takes some of the cream of that crop and creates little comics out of it, lampooning just how absurd the lines are if taken at face value.
The Chongo Show has started a series called "What the Fanfic", which so far only covers a single crossover between Sonic and Pokemon, however he has mentioned featuring one with Loki soon. The readings are blind and the comments/reactions are improvised.
The Livejournal pages heartless_fics and snakesonasora are dedicated to the MSTing of Kingdom Hearts fics. The two most famous fics are "Naga Eyes", AKA "Snakes on a Sora", which has a naga Riku horribly raping Sora, and "Immortality In Fickle Affection", AKA the Kooshball Penis Hydra Fic. More information on both here.
A brand-new Pokémon community was just created recently. It goes by the rather unimaginative name "Sporkémon" and can be found here.
rt_rocket is a LJ that, although originally meant for something else, now does sporkings of Earthbound and Kingdom Hearts fanfiction as riffed by Earthbound and Sonic the Hedgehog fan characters, and sometimes even the MST-writers themselves. Has a lot of injokes and can be a bit confusing sometimes because of up to five people and their characters bickering, and some definite OOC-ness if a canon character joins the ranks, but worth it.
Project AFTER is a site dedicated to ridding the world of terrible fanfiction by MSTing it. However, instead of using a cast of characters, it is just the author of the MST commenting on the work. Its forum also includes a section where fans can post their own MSTs.
The Marvel Comics UK reprint of the comic book adaptation of the 'Transformers'' episode "The Big Broadcast of 2006!" had a Simon Furman framing device in which the Quintessons denied it ever happened.
Deadpool's text boxes have shifted from mere thought boxes to this role in the later years.
The Bongo Comics Simpsons series had a B-story where Bart and his friends turn up for the latest in the Space Mutants franchise, only for the movie to put on an old Troy Mc Clure film called Gladys the Groovy Mule. Bart and friends even cast distinctive silhouettes in the first row while they snark.
In Roughing It, Twain gives The Book of Mormon the same treatment after he is given a copy when he is travelling through Utah.
He does a devastating critique of Christian Science which, along with taking apart its medical and religious principles, just plain mocks Mary Baker Eddy's writing in Science and Health, with Key to the Scriptures.
The Glitterdome is an archive of MSTs for dozens of TV shows, done by taking screenshots and adding captions. It's mostly sci-fi and supernatural shows such as Buffy and Doctor Who.
Hipsoda's Caption Crack is another captioning site, where cappers (as they call themselves) can choose from an assortment of shows, movies, commercials, etc. and even "fave" captions.
Mash did this a few times in its early years when home movies were being screened. Used to its funniest as Hawkeye, Trapper, Klinger, Radar and Henry were watching Frank's home movie of his wedding.
Hawkeye: (as movie first shows line of wedding guests) I've invited you here today as I'm ready to name the murderer.
Game Show Network's original series Faux Pause screened obscure, short-lived game shows, paused them at certain points, and riffed on them.
In Community Abed and Troy apparently have a tradition of doing this to horrendous B-movies; Shirley, Pierce, and Seńor Chang show up, too. (And yet Jeff, played by the one cast member who had guest starred on RiffTrax, does NOT show up.)
Pierce attempts to compensate for his lack of ability at this by writing his jokes with a team of writers beforehand, like the actual Mystery Science Theater 3000 did after its first season.
Some of the DVD Commentary tracks for Mad Men feature the actors riffing on the episodes. It happens pretty much any time Vincent Kartheiser (Pete Campbell) and/or John Slattery (Roger Sterling) are on the commentary, and there are a few episodes where they're together.
FlirtyFroggy wrote a series of commentaries/reviews of Hornblower. Writing and characters are praised very often, but mocked mercilessly when it's deserved. As the author put it: "I mock because I care." It's very snarky, but also includes gushing about Archie Kennedy, because in the author's opinion, he's the cutest and coolest character on the show, and needless to say, this view is mirrored by many fans.
The DVD Commentary by the cast for Farscape often features the cast riffing on the action or dialog. Especially Ben Browder and Claudia Black. Their commentary together on the season one episode "Throne For A Loss" is particularly hilarious.
Comic SJ Perelman would often mock plays or entire novels. He even had a special prefix for the times he mocked a book he had read as a child; "Cloudland Revisited".
The fivers over at Fiveminute.net have some elements of an MST combined with an Abridged Series. It's a bit complicated; read the examples at the works page.
Canadian music channel MuchMusic has Video On Trial, a series which does it to music videos with a bunch of comedians.
Although short-lived, Lynxara's Mystery Wrestling Theater 3000 was a fun series geared towards wrestling fans. Its two best (and most memorable) entries are its longest: "Coming of Munihausen", and the even worse(!) "A sorcerer, a demon, and Emeralds".
William Shakespeare's play A Midsummer Night's Dream ends with a bumbling theatre troupe that plans to perform a version of "Pyramus and Thisbe" for Duke Theseus, his new wife Hippolyta the Queen of the Amazons, and some guests. The play is an extreme example of So Bad, It's Good; meanwhile, the royal couple and their guests mercilessly riff on it. (It should be mentioned that it wasn't taboo to talk during the play in Shakespeare's time; and also that "Pyramus and Thisbe" had been adapted for the stage several times, always poorly.)
Given the precedence, authors MSTing Shakespeare fanfics often use Theseus and Hippolyta.
Just a few years after the Shakespeare example, it was included in Francis Beaumont's The Knight Of The Burning Pestle in the form of a loud middle-class couple that's supposed to be part of the audience. They decide they have a better view of the show if they sit on stage, then get in an argument with the guy reading the prologue, and soon berate him into changing the play so it focuses on the middle class—which, remember, was a pretty new class at the time—and also into making up a new character and subplot so that their servant-boy can be in the show. They never do shut up, occasionally interrupting the action to complain.
In the Commedia dell'Arte, there was a character called the Capo or Capocomico, ostensibly a narrator, but in some shows he would join the action, complain to the audience, and sometimes even freeze the action and physically move the actors.
Broken Pixels was a series on 1UP.com where three guys would play bad video games for comedic value. Featured Seanbaby.
Broken Pixels received a spiritual successor of sorts called Shame Night, a live Ustream show shown every Friday. Unfortunately, that's now gone as well, due to co-host Scott Sharkey being laid off under mysterious circumstances.
They also did a "Two Best Friends Watch" with their friend Woolie of Metroid: Other M through the in-game Theater mode.
Any given Let's Play might be this, depending on the player's opinion of the game and the overall tone of their LPs. Pokecapn and his crew (whether it's all four guys together or just any combination thereof) tend to choose games that are ripe for this sort of treatment, so naturally it's going to happen.
Two notable examples are Brain Scratch Commentaries and Hellfire Commentaries. Hellfirecomms mostly only has two or three commentators (Except in the recent Sonic comms, which have NTom, FTA, Skyler and SonicMovies) but Brainscratch can have anywhere between four to six due to having a main cast of four members.
In Homestuck, Dirk MSTs a Pony Pals book. As the story goes on, he starts going Off the Rails, blacking out and rewriting entire chapters, keeping only the chapter titles. According to Jane, it becomes "a tough, emotionally draining read. But it's cathartic, in all the worst ways possible."
When the Powerpuff Girls were reluctantly forced by the Professor to watch an old silent film in an old revival theater, they began to spout wisecracks and make fun of of the movie. They even sit in the same position at the same silhouette angle of MST3K.