Naritaverse

Naritaverse is the unofficial fan name given to the shared universe created by Ryohgo Narita, author of Durarara!! The Naritaverse encapsulates Durarara!! along with three other novel series by Narita: Baccano!, Vamp!, and Etsusa Bridge.

Baccano!
The earliest entry in Naritaverse, Baccano! tells the stories of various immortal beings and the chaos they bring to themselves and those around them. The overarching story takes place over the course of three main eras: the early 1700s; the 1930s; and the early 2000s. It is the longest running and most expansive series of the Naritaverse.

Vamp!
Taking place in modern-day Germany on the fictional island of Growerth, Vamp! tells the story of vampire twins with unique properties, their bizarre adoptive father, the island's dhampyr mayor and citizens, and many other vampires, humans, and variations of the two that get dragged into the story.

Etsusa Bridge
Chronologically last in Naritaverse, Etsusa Bridge is set in the late 2010s and early 2020s. It takes place in a city built on a man-made island located in the center of a massive bridge between Sadogashima Island and Niigata. The bridge's construction was abandoned last minute, becoming a haven for outlaws and outcasts.

Anime

 * Isaac Dian and Miria Harvent, a pair of eccentric thieves 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), who are actually people covered in glow paint thrown across the sky by Shizuo. (Episode 12.5)
 * In the eleventh episode of Durarara!!x2 Ketsu, Mikado Ryuugamine mentions that the firing mechanism on his hand was designed by a terrorist in America years ago. That terrorist was Huey Laforet, and a prototype of his mechanism was used by Goose Perkins on board the transcontinental express Flying Pussyfoot in December 1931.

Light Novel

 * In volume 4 of Durarara!!, the serial killer known as "Hollywood", who wears various monster costumes while committing murders, is compared to an American couple known for committing robberies in costumes. The couple mentioned are Isaac and Miria of Baccano!.
 * Nile, one of the original immortals from the Advena Avis, somehow got into a fight with Shizuo while on vacation in Ikebukuro in July-August 2002. The fight ended in Shizuo's favor.
 * The unnamed photographer who narrated volume 01 of the Baccano! novels lives in the apartment next to Mikado.
 * Ronny Schiatto, a high ranking member of the Martillo Camorra as well as the 'demon' who bestowed the crew of the Advena Avis with immortality, is aware of Celty Sturluson to some extent.
 * During the modern-day story arc, Bride asks his subordinate to drive him to "Orihara," so he can return a key and thank Orihara personally for taking care of corpse disposal on Bride's behalf. This is an obvious reference to Izaya.
 * On page 36 of Durarara!! volume five, Chikage Rokujou remarks, "Well, the other day I saw a thing on TV about a nobleman who loved, like, thirty different maids..." This may be a reference to Baccano! character Count Esperanza C. Boroñal, who employed over thirty maids during his residency as the acting governor of Lotto Valentino in the early 1700s. Both characters have unique perspectives when it comes to women.
 * In Durarara!! volume 8, Saki makes a comment on how the child stunt actor, Charon Walken, bears a striking resemblance to Kasuka. It's worth noting that Charon and his sister Claudia are the great-grandchildren of Felix Walken (formerly known as Claire Stanfield) and Chané Laforet.
 * A popular director in Baccano!, John Drox, frequently hires Kasuka for his movies. Promotional material for John Drox's movie based off the recently declassified events of the Flying Pussyfoot can be seen throughout Ikebukuro in the Durarara!! anime, in the form of posters and clips from Baccano!'s 2007 anime adaptation (which can be seen playing on the large screen in the square).
 * In chapter two of A Standing Ovation with Izaya Orihara, Yuuki Usubara recounts a story told to her by an American couple touring Japan. The couple may be Isaac and Miria. In chapter three, two characters recall hearing of an American serial killer named Ice Pick Thompson whose identity was never discovered. Ice Pick Thompson is the subject of the sixteenth Baccano! light novel. In the fourth chapter, Izaya briefly mentions the influential Runorata Family mafia, one of the powerful mafia syndicates featuring in Baccano!
 * In Durarara!! SH volume four, a story is told of an American couple who dated for fifty years and went on their honeymoon twenty years after the fact. It is likely that the couple in question is Firo and Ennis Prochainezo, two characters from Baccano!

Anime

 * In Durarara!!x2 Ketsu episode 1, Shingen explains his work with Nebula to Walker, mentioning an "elderly video game-obsessed loli" and a "beautiful hungry-type werewolf girl." These are references to Romy Mars ("The Silver-Wheel Stage") and Watson respectively.

Light Novel

 * Vamp! is one of the only series to have a full on crossover with Durarara!! Appropriately titled "Duramp!," the short story follows a succubus (a demon who feeds off the lustful desires of men) as she journeys to Ikebukuro to feed on the desires of the residents there and to gain enough power to take control of Growerth. Over the course of the story the succubus encounters several characters including Mikado Ryuugamine, Mika Harima and Seiji Yagiri, Shizuo Heiwajima, Walker Yuumasaki, and Celty Sturluson.
 * In the gaiden "The Two Getting Along," a black-hole like being is mentioned seen in a photo of Waldstein Castle. This is referring to the black-hole vampire, Hawking.
 * A famous martial artist in Vamp!, Traugott Geissendörfer, owns a series of martial arts schools across the world. His fighting style is practiced by Mikage Sharaku, and Mairu Orihara learned most of her martial arts skills from the Japanese branch of one of his schools before eventually moving to Rakuei Gym. Shizuo Heiwajima is also a big fan of Geissendörfer, who was invited to speak at Raijin High School's closing ceremony during Shizuo's graduation year. In Durarara!!x2, Shizuo mentions Traugott by name in episode 16. The Raijin incident itself is adapted by Durarara!!x2 Ten episode 1.5.
 * A famous artist from Vamp!, Carnald Strassburg, is also briefly mentioned by Shingen while solving a crossword puzzle that required the artist's name for one of its answers. In addition, a saleswoman attempted to sell a fake lithograph of one of his paintings to Chikage at an art gallery in Ikebukuro.
 * According to Kasane Kujiragi, the design of the demonic hammer Bannajin was inspired by another hammer created by Strassburg, and is in fact a replica of it.
 * In a discussion concerning vampire clans, the Hijiribe clan is mentioned as a small family network that has assimilated into human society. This ties in with Ruri Hijiribe's and Kasane's partial vampirism.
 * Shinichi Tsukumoya, a character who appears in a handful of Durarara!! novels, is one of Izaya's primary information suppliers and serves as a narrator. It is implied that Shinichi does not exist in the real world but only on the internet as a sort of AI. A character in Vamp! called QAWSED ("Hackey Mouse") also claims to only exist on the web as part of a race of beings who live in digital form. It is likely, though not outright stated, that Shinichi is a member of this same race.
 * According to Narita, Manami Mamiya and Masashi "Mage" Mamiya of Vamp! are somehow related, though the nature of the connection is a secret.
 * Byakuyamaru Natsugawara's role model is Rude Gardastance, a Vamp! vampire and former chairman of a large American conglomerate.

Light Novel

 * It is said that Celty and Shinra have something to do with the political scandal caused by the construction of the bridge. This is also mentioned in the x2 Shou OVA.
 * Aoba is seen conversing with an unknown Blue Squares member at various points whose last name is Yatsufusa. This is the surname of the original producer of Buruburu Airwaves. After Yatsufusa's death, Etsusa Bridge character Kelly Yatsufusa took up his disk jockey position and adopted his last name as her own.
 * Souji Kuzuhara, head of the West District volunteer police, is part of the Kuzuhara family of police officers including Kinnosuke Kuzuhara and Shinju Kuzuhara.
 * The Ei family of the Dragon Zombie bosozoku gang is connected to the Ei Family of the island's Chinese Mafia.
 * Yahiro Mizuchi privately shudders at the thought of someone operating on logic along the lines of "I killed because the sky is blue" walking around Ikebukuro. This is a reference to Etsusa Bridge character Yakumo Amagiri, who operates on such logic.
 * In the epilogue of A Standing Ovation with Izaya Orihara, Yuuki Usubara exclaims that she wants to visit the artificial island and "the Etsusa bridge."

Yozakura Quartet
While not part of the Naritaverse, Yozakura Quartet and Durarara!! have connections due to their shared illustrator, Yasuda Suzuhito.
 * A 100-page crossover manga titled Yozakura Quartet x Durarara!! was released as a bonus for the home video release of Yozakura Quartet: Hana no uta. The exact plot details are unknown at this time so it is uncertain if it is considered canon.
 * In volume 3 of the light novels, Izaya instructs Namie to send several documents to a Sakurashin trading partner in his computer's address log.
 * Masaomi Kida is seen singing Sakurashin Town's anthem in one of the chatroom conversations in volume 4 of the light novels and directly mentions Sakurashin Town as the origin.
 * Assuming this is the same Sakurashin Town from Yozakura Quartet, and due to the large demon populace of said town, this reference implies that the existence of demons and other supernatural creatures is common knowledge in the Naritaverse.
 * If this assessment is true, a minor plot hole exists during the first Dollars meeting when Celty reveals her true form. Many people were surprised or even horrified at Celty's appearance. Assuming demons are common knowledge, a headless horseman riding around a city shouldn't come as this big of a shock.
 * It is possible that Sakurashin Town isn't well known to the world at large and as such, while people may know the town exists, its large demon population could be kept secret from the general public. This would make Ikebukuro's response to Celty's nature a bit more believable.
 * Alternatively, it is possible that, while demons are confirmed to exist, they are extremely rare outside of Sakurashin Town and as such it would mean most humans have very little experience with the super natural which, again, makes the public's reaction to Celty more plausible. This also makes Celty's relatively quick acceptance into normal society more believable as, after her big reveal, all that occurs are a few news reports and an increase in police patrols. Were Celty the first supernatural creature to go public, it would have attracted much more attention than a mild increase in traffic patrols.