Clarity Media develops several online puzzle players and tools. One recent development is a one player sudoku based game. Use the rules of sudoku to place valid numbers in the grid, and see if you can beat the computer.
Depending on the difficulty level you choose, the computer is more or less likely to make a mistake. You also have a time limit per move, and things get really tricky when you only have a few seconds to place a number when the grid is filling up. What can you score?