Light Novel: Durarara!!

Durarara is a show about a magical fairy who rides a motorcycle and saves people. This is both the most literal and least representative description of the show that I could think of.

—Disorganization XIII, Spoiler: The Panel 4

Three "ra"s, by the way.

Ikebukuro, a district of Tokyo. A young man named Mikado Ryugamine has moved there from "the sticks", clueless as to how things work in the big city. Reunited with his childhood friend Masaomi Kida for the first time in years, Mikado is shown the way around town as he tries to adapt to the hectic ways of the busy metropolis and discovers its many dark secrets.

Word on the street is that a headless biker called the Black Rider (a modernized Dullahan from Celtic Mythology) has been riding about committing acts of vigilante justice in a spectacularly violent fashion.

There's a guy who dresses like a bartender with inhuman strength, a shady Knowledge Broker, a genius Back-Alley Doctor, and a conspiracy about a pharmaceutical company involved in human trafficking. Then there's the "Dollars", a mysterious "colorless" gang whose members and activities are shrouded in secrecy.

Based on a series of light novels written by Ryohgo Narita, the mastermind behind Baccano!; with art and designs by Suzuhito Yasuda, of Yozakura Quartet fame. It can be read here.

An anime adaptation aired in Japan from January through June 2010, running for 24 episodes, with two additional episodes on the DVD release. The anime adaptation was directed by Takahiro Omori and animated by Brain's Base, the same studio/director combo that adapted Baccano!. Aniplex picked up the series for North American distribution, and the DVD releases were done in three groups in January, March, and May 2011. The show aired on [adult swim] starting June 2011.

A second season was announced as part of the 10th anniversary of the franchise at the Dengeki Game Festival in March 2014. The original cast will be reprising their roles. A brief promotional video can be found here.

There is a crossover light novel with A Certain Magical Index called A Certain Magical Index VS Durarara!!