The G8 (ジーエイト, Jī Eito) or God 8 (ゴッド8, Goddo 8) is the name given to the 8 people who have held the title of Shaman King.
About
If 3 or more members raise a complaint against the current Shaman King, an event called the G8 Summit is held in the highest society of the G.S. (where Hana went when he died as a child). The fate of the planet is decided here.
Members
Anime/Manga Difference
Trivia
Just speculation, but it seems like these G8 might be, or at least be based upon, some historical deities. Those whose identities haven't been revealed yet, might be Jesus Christ , Siddharta Gautama , Zeus and Ra .
References