"Hobo nickels" are made from US Buffalo nickels. They are made by carving a new face from the Native American face. During the depression, artists would carve these nickels and sell them for a profit (hence the name "hobo nickel.") Neatorama offered up this example of a more recent hobo nickel, which resembles a science fiction interplanetary humanoid. See more alien head nickels here.
Mark Frauenfelder is the founder of Boing Boing and the editor-in-chief of MAKE and Cool Tools. Twitter: @frauenfelder. Come and hear Mark speak at the ALA conference in Chicago on July 1.