Variants

Sidokus are musical sudokus, with musical notes instead of numbers and a hidden melody.

3D Sudoku program for Windows and Mac. Easy puzzles are free, others cost money.

Many free puzzles playable online, in three sizes: 4x4, 6x6, and 8x8, in three difficulty levels. Also has "no-op" puzzles (where the operator clue is not given).

Puzzles playable online, sizes 3x3 to 9x9, as well as tutorials and tips.

Comprehensive site with Killer Sudoku puzzles, variants, and solving strategies.