This is a duplex nail.
It's called that because it has that little mutated head on top of it, so that it has double heads. Kind of freaky. Duplex nails are great for when you need to do something temporary, because you just nail it in up to the first head and can use the second head to pull it out later. Like, for instance, if you're cutting a board that is as long as your room and have to stick it through the wall for balance!
You just nail a piece of scrap wood to the studs with duplex nails at the appropriate level, and boom! Instant -- and temporary! -- sawhorse. Fantastico.