Ghost (chess)
Ghost is an amateur chess program or more specifically a chess engine, because it has no own graphical user interface (GUI).
It is free software (in the sense that it doesn't cost money), but it's currently not open source. As other chess engines like GNU Chess, it can be used in conjunction with a GUI program such as XBoard.
It is under development since about 2000 and uses common techniques like bitboard-representation of the board and a variation of the alpha-beta minimax algorithm combined with an evalutation function and a transposition table to store previously searched positions.