Kida's four friends were named after a collection of writers and their works in "Dengeki Bunko" (the publisher of the "Durararara!!" comic): Kyohei Kadota was named after Kadono Kohei; Walker Yumasaki was named after Masaki Okayu; Erika Karisawa was named after Erika Nakamura and Mamizu Arisawa; Saburo Togasa was named after Soichiro Watase.
The password for the Dollars' website is "Baccano!", the title of another series Ryôgo Narita worked on. References to other Aniplex series are found in many places in Durarara!!, with Baccano! and Darker Than Black being especially prevalent.
The trivia items below may give away important plot points.
Izaya Orihara's name combines the Japanese pronunciation of Isaiah ("Izaya") and the Japanese term for "the one who looks over the crowd" ("orihara"). The last term refers to him having set in motion most of the events in the show, and then sitting back to watch what ensues.