Not so much a form of Fanfic as a style of commentary on another's Fanfic, usually tongue-in-cheek but sometimes quite vitriolic.
Most writers of fanfic post their works in venues where they hope to receive feedback in order to improve their skills. The MST is a specific variety of story commentary based around the format of Mystery Science Theater 3000: three or four characters "watch" the fic unfold, and make snarky and/or constructive comments on it. The usual tone is one of lampoonery, sometimes verging on ridicule if the story deserves it. Extreme examples can mimic the entire structure of an episode of MST3K, right down to the message from the Mads and the invention exchange.
Originally, MSTers used the actual cast of MST3K, but as time went on using alternative casts became more and more popular. More often than not, alternative cast members come from the series the Fanfic being MSTed is based on, although they're usually something completely different — popular anime characters, professional wrestlers, a Cross Over of several different series, or even original characters created for the role. The setup for alternative casts will usually mimic the "mad scientists inflicting it on a trapped audience" format of MST3K.
Usually, the format goes like so:
> Line from the original fanfic
CHARACTER: Snarky comment!
(Incidentally, the format for lines from the original fanfic likely has to do with modern fanfic's Usenet and internet-mailing-list origins; the > is used at the start of each line quoted in a post in old e-mail programs.)
Naturally, some MSTings get MSTed again, a practice known as "meta-MSTing". (Compare with Retsupurae).
An MST takes quite a bit more work than the usual comments and criticism a story may receive, and is usually reserved for stories that are either unusually good or extraordinarily bad. Because of its often inflammatory content, the creator of an MST will usually get the permission of the author first.
The practice isn't always limited to Fanfic, either; for example, particularly annoying spam may be given this treatment. Another common class of targets are posts espousing off-the-wall conspiracy theories. Obviously, the permission of the author isn't sought in these cases. When done in political commentary, this is called Fisking.
The practice is known as MSTing or misting (not to be confused with Myst). In some fandoms, however, it is known as a sporking.
While the practice (and the community surrounding it) has waned in recent years, you can still find new MSTings floating around if you know where to look.
For the more traditional snarking of bad movies MSTing, see Alternate DVD Commentary.
Compare The Heckler.
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.
A non-fanfic example: Jeff Vandermeer's novel Shriek: An Afterword features the story's protagonist snarkily commenting on the narrator's work.
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.
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 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).
M*A*S*H 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.
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".
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 Pokemon Attack Of Mewtwo and its sequel.
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 MS Ts, he has My Inner Life, a Sue-fic from Kingdom Hearts, and a couple of more recent fics he's found.
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.) Authors MSTing Shakespeare fanfic often use Theseus and Hippolyta.
Just a few years later, it was included in "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.
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.
But now, Retsupurae started making fun of bad games in longplay videos.
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.
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 N Tom, FTA, Skyler and Sonic Movies) but Brainscratch can have anywhere between four to six due to having a main cast of four members (Or three, in the days before Ted joined)
A number of webcomics MSTed themselves in the 2001 MST3Keenevent. (Note that most, if not all, of the links on that page are defunct.)
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.'
"Faps" erases most of the text from bad Yaoi Genre webcomics or mangas and replaces it with either completely random text or commentary from the characters. The text that isn't erased always has a riff (which these are referred to as) and there's always mention about the bad artwork.
Another non-fanfic example: the Whateley Universe now has a story "Tales of the MCO", in which the students are watching the aforesaid anti-mutant show and MSTing it the whole time.
The Kimbettes MSTed the first assembly of the Fall '06 term.
What do we do with witches?
We burn them! Sorry Nikki!
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 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 Maclure film called Gladys the Groovy Mule. Bart and friends even cast distinctive silhouettes in the first row while they snark.
Mystery Fiction Theater N is currently riffing Faithful Hearts by legendary Suethor DisneyFan01. They may eventually do the retreaded version Faith of the Heart, but haven't decided yet.
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.
My Immortal gets riffed by ponies. Obscenites (except for misspells and "WHAT THE HELL ARE YOU DOING YOU MOTHERFUKERS"), and the sex scenes are removed.
The MLP/MST Archive is a relatively new MST archive that collects various riffings of My Little Pony fanfics, including ones from Fan/fic/Theater 3000 and the now-defunct Mystery Pinkie Pie Theater 3000. Visitors can also submit riffs of their own to the archive for safekeeping.