The well-known mastermind game.
Drag-and-drop colors from the palette or from the board itself. The
object is to determine the hidden combination of four colors. The game
may be played in two modes: with replacement, which means that colors
may repeat, and without replacement, which means that colors are
unique. The user selects a choice of four colors and "commits" them.
The program replies with an evaluation -- a black peg for a color that
is placed correctly, and a white peg for a color that is in the wrong
position. The user may make a total of eight queries.

WWW: http://www.gnustep.it/marko/GMastermind/