I just finished watching all 24 episodes of Durarara!!. It's a very interesting anime. Its a story of how different lives are intertwined, whether you're human or otherwise. There were quite a few characters that it took almost the first half of the series just to show the background story of each one of them. It also has a number of interesting twists. At least it got me hooked enough to marathon the whole thing.
The characters were all different; Celty (the headless rider), Mikado (founder of Dollars), Masaomi (shogun of the Yellow Bandanas), Anri (the Slasher), Izaya (information dealer), Shizou (the strongest of fighter), Simon (Russian Sushi seller), Shinra (underground doctor)... and a whole lot more.
Most of the reviews about Durarara!! around the web say that its predecessor is better, Baccano! I can't tell yet because I haven't seen it, yet.
Next up, search for Baccano!.