Shaman King is a Japanese manga
written and illustrated Hiroyuki Takei. It has been running since 1998, and is published in Weekly Shonen Jump in Japan, as well as Shonen Jump in the USA.
Shaman King chronicles the adventures of Yoh Asakura, his geeky friend Manta Oyamada, and his ghost samurai companion Amidamaru. Interested yet? Yoh is a young shaman, a person who can see and hear ghosts, and also let them possess his body. When a shaman lets a ghost do this, he gains all of the ghosts power and knowledge...which makes for some superhero-style action.
Yoh has come to Tokyo to gather new, powerful ghosts and finish his shaman training. There he meets Manta, a short, somewhat neurotic boy, who happens to also be able to see ghosts, and Amidamaru, the ghost of an infamous samurai. Manta gets into trouble, Yoh saves him with Amidamaru's help...and the adventures begin.
Along the way, the trio meet other shamans, including the obsessed Ren, possessor of a massive superiority complex, and his sister, Tao Jun, who can control ghosts...and their corpses. From Ren they learn about the Shaman King, a shaman able to merge with the greatest spirit of all...God. Ren wants to become Shaman King so he can rid the world of those he deems inferior. Yoh decides he wants to become Shaman King so he can take it easy.
Thus they are launched on a series of adventures and battles, all stunningly yet simply drawn. Takei's sense of humor and excellent battles keeps Shaman King drawing you in. Currently Shonen Jump magazine is running the first volume of the manga, with plans to continue the series if it is popular.
Note: As I don't have access to the full series of Shaman King so far, I will be updating this as more of the story is translated into English.