The Masterminder solver can be launched with the following button. You can also guess yourself, which is more fun to play, but less of an exciting computational problem:
The allowed colors are empty, blue, red, black, white, green, and yellow. For a description of the rules of Mastermind, see here. The solver computes the question with the maximum informational entropy and asks this question. The code is provided here.
I originally developed the code in Java in 1997.