Naritaverse

Ryohgo Narita, Durarara's author, has stated that almost all the novel series he has worked on take place in a shared universe (referred to as the "Naritaverse" by fans). The series that most frequently cross paths with Durarara!! as well as each other are Baccano!, Vamp!, and ''Etsusa Bridge. '' This page is dedicated to listing specifically durarara's connections in all three mediums: Anime, Light Novel, and Manga, to each of these series.

Please note that due to the relative scarcity of info on Narita's novels outside of Japan, much of this information is subject to change as new facts come to light.

Anime

 * Isaac Dian and Miria Harvent, a pair of eccentric immortals from Baccano, are present at the first Dollars meeting in Ikebukuro. (Episode 11)
 * Isaac and Miria are also seen discussing "Japan's ghostly fireballs" (Hitodama in the original sub), actually people covered in glow paint thrown across the sky by Shizuo. (Episode 12.5)

Light Novel

 * Nile, one of the original immortals from the Advenna Avis, somehow gets into a fight with Shizuo while on vacation in ikebukuro. The fight ends in Shizuo's favor.
 * The unnamed photographer who narrarated volume 01 of the Baccano novels apparently lives in the apartment next to Mikado.
 * Ronnie Sukiart, a high ranking member of the mafia as well as the demon who bestowed the crew of the Advenna Avis with immortality, is apparently an aquantance of Celty's. However, the details on how they met aren't known.
 * During the modern day story arc, Boss Buraido mentions "Orihara of Shinjuku" as one of his primary information sources. This is an obvious reference to Izaya.
 * Izaya also appears during the aftermath of the hijaking of the cruise ship The Entrance as it docks in Japan. Izaya assissts Buraido in disposing of the bodies of some of the hijackers.
 * In volume 08 of durarara, Saki makes a comment on how the child actor, Charon Walken, bares a striking resemblance to Kasuka. It's worth noting that Charon, along with his sister Claudia,  are the great grandchildren of Felix Walken (formerly known as Claire Stanfield) and Chane Laforet.
 * A popular director in Baccano!, John Trox, frequently hires Kasuka for his movies.

Light Novel

 * To date, Vamp! is the only series to have a full on crossover with Durarara!! Appropriately titled Duramp!, the shortstory follows a pair of Succubi (demons who feed off the lustful desires of men) as they journey to Ikebukuro for their next meal.
 * The first succubus visits Walker in his sleep and shapeshifts into one of his greatest fantasies, a green haired maid. The next day, Walker attempts to tell his friends about how said maid had visited him the night before but everyone else just dismisses it as his usual ramblings.
 * The second Succubus chooses Shizuo as her next target and decides to take the form of the most prominent person in his mind. Unfortunately for her, at the time, Shizuo had just gotten out of another fight with Izaya and the encounter was still fresh in his mind. Mistaking Shizuo's blind hate for blind passion, the succubus takes Izaya's form and attempts to flirt with him. Thinking Izaya had returned for round 2, Shizuo savagely beat the succubus into the ground and went home. The succubus, meanwhile, returned to her previous city horrifically traumatized.
 * A famous martial artist in Vamp!, Traugott Geissendorfer, owns a series of martial arts schools across the world Mairu Orihara learned most of her martial arts skills from the japanese branch of one of his schools before eventually moving to Rakuei Gym. On a side note, Shizuo is also a big fan of Geissendorfer.
 * A famous artist from Vamp!, Karnard Strassburg is also briefly mentioned by Shingen while solving a crossword puzzle that required the artists name for one of it's answers.
 * Tsukumoya Shinichi, a character who appears briefly in a handfull of durarara's novels, acts as one of Izaya's primary information suppliers. It is implied that Shinichi does not exist in the real world and only exists on the internet which makes him a sort of AI. A character in Vamp! called Husky Mouse also claims to only exists on the web and is part of a race of beings who only exist in digital form. It is likely, though not outright stated, that Tsukumoya is a member of this same race.
 * On an unrelated note, Tsukumoya is probably one of the only people that can genuinely get Izaya upset just by talking to him. This is primarily due to the fact that Tsukumoya always seems to have the upper hand on Izaya when it comes to gathering information.

Light Novel

 * It is said the Celty and Shinra have something to do with the political scandal caused by the constuction of the bridge.
 * Aoba is seen conversing with an unknown individual at varying points in the novel. This person is named Yatsufusa who is one of the main characters in Etsusa Bridge.
 * Kuzuhara Souji, the youngest son of Kinnosuke Kuzuhara, is a police officer stationed on the City built on top of Etsusa Bridge.