Gantz (ガンツ, Gantsu^?) is a manga and anime series written by Hiroya Oku. Gantz tells the story of a teenager named Kei Kurono who dies in a train accident and becomes part of a semi-posthumous "game" in which he and several other recently deceased people are forced to hunt down and kill aliens. The missions in which they take place are often dangerous, and very few members of the team survive; they are replaced by others who have recently died.
[source http://en.wikipedia.org/wiki/Gantz]