Everybody should have this show on their top 5 anime list
Some character designs are based on Lupin III, Spike -> Lupin and Jet -> Jigen. The final scene with the shot couple in the first episode, was taken from "Bonnie & Clyde". The shooting at the bar was taken from "Desperado" with Antonio Banderas. Spike was also based on Bruce Lee, so there are a lot of references to his films. "Way of the Dragon" when Spike uses the nunchakus in the second episode. The homage to Miyazaki and his "Porco Rosso" in episode 19 (Wild Horses) was very fun
Those are some of the hundreds of examples!!
There are a lot of references to films, other anime, music bands in this series, definitely one of the best anime series ever!!!