is a free Mac Sudoku program. It can generate puzzles and puzzle books; it can solve puzzles you get from elsewhere, and it can give you meaningful hints as you are solving a puzzle based on your progress so far.
It can even generate 4 by 4, 16 by 16, and 25 by 25 puzzles by changing the preferences, but be careful because it's slow to generate big ones.
Try it. You'll like it.