KenKen is a great Japanese math game similar to Sudoku that is supposed to train your brain to do simple math quickly.

The rules are simple:

As in sudoku, the goal of each puzzle is to fill a grid with digits - 1 through 4 for a 4×4 grid, 1 through 5 for a 5×5, etc. - so that no digit appears more than once in any row or column. Grids range in size from 3×3 to 9×9. Additionally, KenKen grids are divided into heavily outlined groups of cells - often called “cages” - and the numbers in the cells of each cage must produce a certain “target” number when combined using a specified mathematical operation (either addition, subtraction, multiplication or division). For example, a three-cell cage specifying addition and a target number of 6 in a 4×4 puzzle might be satisfied with the digits 1, 2, and 3. Digits may be repeated within a cage, as long as they are not in the same row or column. No operation is relevant for a single-cell cage: placing the "target" in the cell is the only possibility. The target number and operation appear in the upper left-hand corner of the cage.

Try it here, and keep coming back for new puzzles every day.