Comiket

Comic Market (コミックマーケット, Komikku Māketto), more commonly known as Comiket (コミケット, Komiketto) and sometimes Comike (コミケ, Komike), is a biannual dōjinshi fair in Tokyo, Japan. A grassroots, DIY event focused on the sale of dōjin (self-published works), Comiket is a not-for-profit, volunteer-run event administered by the Comic Market Preparatory Committee (ComiketPC). Inaugurated on 21 December 1975, with an estimated 700 attendees, it has grown to become the largest fan convention in the world, with an estimated attendance of over half a million.

The convention takes place twice a year, at the Tokyo Big Sight.

Due to most mangakas being manga fans themselves, and many of them having started in the fan circuit, any work with Otaku themes will feature either the real deal or an Expy of it

Appearances and Mentions in Fiction:

As Comiket

Anime and Manga

Hanaukyo Maid Tai, episode 5.
Lucky Star, episode 12 - Konata complains that it's changed from a friendly get-together into a marketplace over the years. This little adventure provides the page image for Doujinshi.
Nurse Witch Komugi: Koyori turns the Tokyo Big Sight into a giant mecha during Comiket.
During episode 7 of Miss Kobayashi's Dragon Maid, Makoto sells his dōjinshi at Comiket... where Kobayashi and Tohru discover that some of the cosplayers aren't actually cosplaying.
Narumi sells a yaoi doujinshi at summer Comiket in episode 3 of Wotakoi: Love Is Hard for Otaku
Given that the title of episode 4 of Oreimo is "There's No Way My Little Sister Would Go To Summer Comiket", guess where they end up going.

Web Comics

Hang In There, Kogasa-san, being about mangakas / doujinkas, have several episodes of the characters going to the Comiket either as fans or as circles with their own table.

As a Bland-Name Product

Anime and Manga

Comic Party uses "ComiPa" to drive its plot.
Genshiken has "ComiFes".
Kyōko, Yui and Chitose drag Ayano to "Comuket" in episode 5 of Yuru-Yuri.
Episodes 9-12 of Keep Your Hands Off Eizouken! have the animation club strive to complete and sell an anime at "Comet A".
"Harumi in Summer" from Otaku no Video

Web Original

Fenspace has a "Cosmic Party", based on the Comic Party anime and game that reference Comiket.

Other References

Anime and Manga

When aliens steal the Tokyo Big Sight in Mao-chan, the building is described as the home of a convention loved by otaku instead of being named.