This sudoku solving program fills in 9x9 puzzles. Some puzzles can have several solutions. The software calculates one possible solution. Even entering a number is enough to start. Copy the numbers to the appropriate locations and press 'Solve Sudoku'. It takes less than a second to get the solution.
If you don't know the rules of sudoku you can read it here. You can practice on the page 'sudoku for kids'.
As you progress, you may try the easy, medium, hard and super versions.