Gomoku is an ancient Japanese strategy game (also called Five In A Row). Its rules are very simple - each of the two players (Black and White) place a stone alternately in a square on a 15x15 grid. The goal is to get five unbroken stones in row - horizontally, vertically or diagonally (like an extension of Tic-Tac-Toe).
You can play against the computer or against another human.