Dr. Sudoku

Sudoku looks like a math problem, but it's not. The rules are simple: start with a grid consisting of nine three-by-three squares in which some of the numbers are already supplied. Fill in the blank squares so that each column, row, and three-by-three grid contains a number from 1 to 9 with none repeated. The game features hundreds of puzzle problems, a tutorial mode, help for those moments when you really are stuck, the ability to 'pencil in' possible solutions, and a puzzle problem creation mode.