Baccano! — due to the series having the same author and are both supposedly taking place in the same universe, there are multiple references to it:
Any time someone logs on to the Dollars web site the password is "Baccano".
Episode 1: A clip of Jacuzzi appears on a Jumbotron.
Episode 3:
Several pictures of the main characters appear in the background.
When Mikado and Anri flee the Shizuo/Izaya showdown, the way the camera focuses on them Holding Hands and the way Anri ends up half-dragging Mikado bears a strong resemblance to Nice and Jacuzzi in the Baccano OP.
Episode 6: A Baccano billboard is featured in the end of the episode.
Episode 7: Yet another Baccano poster is seen.
Episode 8: "Does the scene where Walker and Erika are dancing in the street remind you anything, Miria?" "What, Isaac?
Episode 11 and 12.5: Issac and Miria make a cameo during Mikado's Moment of Awesome. They are seen again in the beginning of episode 12.5.
Episode 12.5: A poster of Claire Stanfield can be seen in several parts.
Episode 18: Yet more Baccano clips are shown on a tv screen in one scene.
Episode 19: A poster of Chane Laforet can be seen in the background about halfway in.
Episode 24.5:
A huge billboard containing many of the major characters can be spotted around the beginning.
Some Baccano clips are also shown in the Jumbotron.
A large poster of Ennis is seen during Shizuo's fight with Izaya.
The 1st opening has a billboard of the entire cast seen after Celty's introduction.
The 2nd opening has the cover of the 1st light novel seen during Shizuo's introduction.
While the second season's references to Baccano had dialed down, there were still a couple of them:
Episode 1: Issac and Miria briefly appear in a poster in Yuhei's agency.
Episode 18: A clip of Eve Genoard's meeting with Issac and Miria appears as Masaomi walks around Ikebukuro.
In Ketsu's opening, we briefly see a poster of Firo and Ennis during Erika's and Walker's introduction.
Episode 1: Walker is carrying a cardboard cutout of Holo.
Episode 3:
"They're pipiru piru piru angels who fly in the sky while showing their underwear to everyone below them!" Also, Walker is carrying a volume of Yozakura Quartet (Suzuhito Yasuda did the character designs for Durarara!! as well). Also: Flame Haze.
Shizuo also gets his face covered in blood lending him some semblance to the Rail Tracer.
Episode 5:
Karisawa & Yumasaki: "The Mellow Scarves?" Kadota: "Yellow Scarves!" Karisawa & Yumasaki: "Sou sore!" (Yeah, that's it!)—Karisawa points out that that was Hermes' line.
One of the various silly claims Teen Idol Kasuka Heiwajima's manager makes about him is that he's a thousand-year-old vampire. Yes, Shizuo's brother is the GACKT of the Durarara universe.
Episode 20: The gang wars are illustrated using a Taiga figure, a Holo figure, and a Shana figure. There's also a Hetalia: Axis Powers gashapon machine in the background, a Hiiro Yuki art book, as well as a clip from Black Butler II on a Jumbotron.
From Volume 4: "Tsun tsun dere tsun dere tsun tsun...Kids sing it around the commercial street of Sakurashinmachi." Evidently, Kida reads Yozakura Quartet.
In the first volume of the manga there is a Grellevator.
In the manga, Erika and Walker use a Black Butler manga as inspiration for their torture. This is most likely because both mangas are being published by a magazine under Square Enix.
In the manga when Mikado and Masaomi go to Animate, keep an eye out for the semi-obvious Fullmetal Alchemist and blatantly obvious Saki references. "A mah-jong manga for girls?"
In Season 2, Episode 8, Walker is shown to be a fan of a female painter named "Suzie Yasuda", a nod to Suzuhito Yasuda, the creator of Yozakura Quartet and illustrator of the Durarara light novel.
Season 2 episode 11 namedrops Dynasty Warriors, courtesy of Walker.
In the same sequence he pulls out a lighter featuring Komoe-sensei and makes a quip about he didn't learn what he's doing from her classroom.
Walker in season 2 episode 12: "I can't believe Anri is a Flame Haze!"
In the episode 31 of the second season, Togusa asks what is going on because of the mess happening on the Dollars message board. Walker responds that they will finally see the advent of the demon city Ikebukuro, that a demon-summoning app will be installed on his phone and their survival life will then begin. And ends the scene with a "Let's survive!", arc words of the game and the anime. Even better because the game itself had a cameo from Durarara characters (due to having the same designer) with Celty, Shizuo using his trademark pole as a weapon, and Izaya in it.... in Japan. So it's a cross-reference between works.
In the English volumes 1 and 2 of Durarara!! SH, Rira talks like Yoda on her site. Nobody from in-universe social media likes it and it's frequently complained that her typing gimmick is crap.
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