In Greek mythology, Charon was the ferryman of the dead. He was a spirit who served Hades, taking the shades of the dead from Akherousian mere to the land of the dead on the other side of the river. Hermes, the messenger god, would gather the dead from the upper world and guide them down to the shores where Charon would then collect them.
For his service, a single obolos coin was paid, which would be placed in the mouth of a corpse at the burial. Failure to do so resulted in the spirits endlessly wandering the upper world as ghosts. [As requested by the-purblind]