Fan Convention

"Rupert: Hobbits will be mustering under Gandalf as usual in the Ops Room. Esoterica with the Master Mage is in a dimension yet to be fixed... Filking will be in Home Universe this year... ...Stan, who are these people? Stan: Ordinary people having fun, I expect."

- Deep Secret

Or just Convention or Con. This is where fans of a particular work or medium all meet together, discuss their show(s) and ask questions of the stars. A number turn up in costume. Frequently parodied in the case of a Show Within a Show, where in(s)ane questions to stars are common.

The largest fan convention in the world is Comiket, the twice-annual doujinshi show in Tokyo. Other famous cons include the San Diego Comic-Con (originally covering Comic Books, but has grown to also cover comic-book-related media such as superhero films) and Gen Con (the oldest and largest Tabletop Games convention).

Anime and Manga
Comiket has its own page.
 * Drama Con takes place at the fictional "Yatta!con".

Comic Books

 * In the Sandman series, a hotel puts on just such a convention...for serial killers.

Fan Works
"Florence: I activate Dark Sanctuary! Now the duel will take place in a twisted and horror-filled environment where only the bravest souls dare to venture. Yami Yugi: An anime convention?"
 * From Yu-Gi-Oh!: The Abridged Series:

Film

 * Galaxy Quest had a major, mostly affectionate, parody of this.
 * The documentary Trekkies touched on the subject.
 * The Bag Witch Project, a Blair Witch parody short film by Toxic Bag Productions, tells the sorry tale of three attendees at GenCon, who get hopelessly lost in the hallways of the Milwaukee convention center.

Literature

 * Sharyn McCrumb's novel Bimbos of the Death Sun sets a murder mystery at a con, where a popular but volatile author is murdered while writing his latest novel.
 * Most of the Diana Wynne Jones book Deep Secret takes place at a sci-fi/fantasy convention. Of course, there are real wizards there.
 * Proven Guilty takes place in part at a horror movie convention.
 * Night of The Living Trekkies is a book about a zombie outbreak at a Star Trek convention; there is a trailer for the book here.
 * Lost at the Con is about a Gonzo Journalist sent to cover Griffin*Con, a thinly-veiled Expy of Dragon Con. As very much a not-geek, his articles (which punctuate the act breaks) start out being very disparaging of the fans, the guests and the con itself,.

Live-Action TV

 * One episode of CSI involved a Furry convention.
 * Vanity Fair and MTV also had hilariously misinformed "documentaries" on the Furry fandom, complete with supposed explorations into a furry convention.
 * Conventions and CSI seem to go well together; aside from the Furry Convention, they also had episodes involving a convention for midgets and one for large people.
 * And one for Astro Quest, the Fictional Counterpart to Star Trek.
 * Doctor Who has a fair number of these, with the Outpost Gallifrey "Gallifrey One" conventions being fairly well known. Peter Davison turned up to a few in his Fifth Doctor costume, before deciding that since others were doing it, it was a bit odd and stopping. Of course, Patrick Troughton famously died at such a convention.
 * The episode "The Real Ghostbusters" of Supernatural has Sam and Dean attending a convention for fans of the novel series based on their lives, which most fans assume is fiction. Hilarity Ensues.

Video Games

 * In Metal Gear Solid, Dr. Hal "Otacon" Emmerich's nickname comes from the real-life Otakon convention.

Web Comics

 * Narbonic inspired both fictional and real-life examples.
 * Within the comic, Dave Davenport travels to Dave-Con, the annual convention of the Dave Conspiracy (every member wearing a badge that reads, "Hello, my name is DAVE").
 * In Real Life, the readers of the comic organized Narbonicon, an annual event celebrating the comic and its author, Shaenon Garrity. It was held during the six years of the comic's original run, but discontinued once the "Director's Cut" version (*cough*reruns*cough*) began.
 * El Goonish Shive had a non-canon side story that took place at San Diego Comic Con.
 * The first Umlaut House comic had Evil Con, for mad scientists.
 * Insecticomics includes several instances of characters (actually, the author cosplaying) attending the real-life BotCon.

Web Original
""RUN! AAAAAAAA!""
 * That Guy With The Glasses runs into a convention in pursuit of Casper the Friendly Ghost, during the Nostalgia Critic's review of the movie Casper. Variously-costumed conventiongoers help him with the chase.
 * Chester A. Bum and 90s Kid attend Youmacon once, where Chester discusses yaoi with a Chester cosplayer, 90s Kid has a 'Dude-off' with a 90s Kid cosplayer, and they interact with other con-goers.
 * In a later video, Chester and Lester A. Bum gather con-goers at Daisho Con 2010 to re-enact Harry Potter and The Deathly Hallows Part One.

Western Animation

 * In an American Dad episode, the Villain of the Week is hiding in a convention, and the only way to get to him is using Steve´s proficiency in elven language.
 * Freakazoid! chased Caveman and was chased by Fanboy during a convention.
 * The Caveguy episode was notable, as Freakazoid talks to the fans in lieu of his creator, and scares Caveguy off by speaking Klingon to him
 * In an earlier episode, while trying to escape from Fanboy (who was trying to become his sidekick), Freakazoid fell into a sci-fi convention. He then finally got Fanboy off his back by pointing out Mark Hamill and tempting Fanboy with the more glamourous prospect of Jedi Knighthood.
 * The Simpsons had Homer saving Mark Hamill from psychotic fans.
 * The battle in Robot Chicken between Star Trek and Star Wars fans at a scifi convention.
 * Phineas and Ferb has an episode set at a "Sci-Fi and Fantasy Convention," with Phineas being a Sci-Fi fan, Ferb being a Fantasy fan, and Candace being an Otaku for cutesy Japanese character Ducky Momo. The Sci-Fi and Fantasy sides actually get into a war, until Phineas and Ferb come up with a plan to remind them that that they're all nerds there, and that Hate Dumb is stupid.