Elementary math teachers will love Tic Tac 15, freeware for Palm handhelds from Nick Robins Software. It's a game that combines Tic Tac Toe, addition, and probability.
The objective of the game is to complete a column, row, or diagonal so that three numbers add to 15. Play can be against the computer or against another person (on one handheld). Players take turns revealing a random number 1-9 and then placing that number on the grid. Since each number is used only once, smart players take note at what numbers have already been used and consider what numbers remain.
Students get to practice plenty of mental math as they calculate the various possibilities before placing their numbers on the grid. Of course, there's also the luck of the draw since the numbers are generated randomly. Tapping the Show button displays the sums of each row, column, and diagonal. With sums showing, there's not as much mental math involved, but subtraction then becomes the skill players practice.
A tally along the right side of the screen keeps track of how many games are won by each player. To keep things fair, Tic Tac 15 players must take turns going first each new game. I suggest posing this question to students: Is Tic Tac 15 a fair game if the same player always goes first? In order to test this, players will need to use "Start Over" from the Options menu after each game. They will also need to keep track of their own wins, as Tic Tac 15's tally count will be reset each time "Start Over" is selected.