Durarara

=Cameoed= Spice and wolf:holo in episode one

Baccano!
In a Baccano! light novel, a character mentions an Orihara who will deal with corpse disposal.

=Cameos=

Baccano!
There are also tons of characters from Baccano scattered around posters or television screens around Ikebukuro.

Black Butler
About 15 minutes into episode 20, a scene from Black Butler II is playing on a screen in the background.

In episode 11, Walker's ringtone is Sebastian Michaelis voice actor, Daisuke Ono, saying "Young master, you have mail".

Others?
"Baccano" is the password to the Dollars website and some of the big screens play footage from Baccano. Isaac and Miria make a couple of cameos. four friends carry around a life-size Holo cutout? plastic figurines, including Taiga from Toradora and Shana Shakugan no Shana? Otaku in OVA says Ryuuji and Taiga best couple? Reference to Kino no Tabi?