Tic-tac-toe is a simple game (usually played with pencil and paper) that can be enjoyed by kids as young as three. What I find most fascinating about these online tic-tac-toe games, is their variety. The many creative twists that can be put on such a simple game are amazing. [Editor's Note: An updated version of this topic can be found here: Tic-Tac-Toe]
This game is unique because it lets you customize the dimensions of your board. You can choose from the traditional three-by-three, or any size up to seven-by-seven. Of course, increasing the size of the board, increases the difficulty of the game. Other choices include who goes first, and whether you want to be X or O. And for those interested in programming, the author provides a behind-the-scenes look at the game's decision-making logic.