The Sudoku Susser is a free program that helps you play Sudoku. It makes it easy to make your selections as you solve the puzzle; just click on one of them and select the new number from the popup menu that appears, or mouse over it and press the number on the keyboard. But it does far more than that!
If you make a mistake and enter an invalid move, the Susser will hilight the invalid squares in red. If you'd like some extra help keeping track of what's what, checking Show Hints will display all the remaining possibilities in the corner of empty squares, and hilight empty squares that can only be a single value in green.
If you want help with a puzzle, the Deduce! button will apply 10 increasingly sophisticated techniques in order to progress. Sudoku Susser can solve all know sudokus by logic, using only techniques that human players would reasonably be able to execute (though a couple are ones that only a Sudoku maniac would try to use). Each step in the solving process is explained in detail. You can single-step, and you can brute-force recurse to get the solution.
With the addition of the “Trebor's Tables” deduction algorithm, the Susser can now logically deduce the solution to the most difficult known Sudoku puzzles.