The Bollard. It looks a bit like a pawn and is placed somewhere on the board at the start, and simply stays there. It never moves, it can't be taken, it just sits there.
(There is also a variant, the Fire Hydrant, which is like a Bollard but yellow, and has a built in water-supply. It can't be used in Computer Chess because only Humans can recognise it)
The detective. Must find out, after the game, who mated the King. Pretty useless during the game.