Ultimate Tic-Tac-Toe takes the classic game of Tic-Tac-Toe and adds a strategic layer to it.

Each square of the 3×3 game board contains another, smaller, 3×3 game board, where you make your move in a square of any small board, you send the opponent in the respective square of the big board.
3 in a row in a small board wins the small board, 3 small boards in a row win the game.

(This is a 2 player game)