From Petr Lastovicka:
You must fill in the symbols 1 through 9, so that each row, column and nonet contains every symbol exactly once. You can also choose letters or colors instead of digits. Board size can be from 4 to 20 (default size is 9). You can play many variants - Killer, Greater Than, Consecutive, Odd/Even, Diagonal, Double, Triple, Butterfly, Flower, Windmill, Samurai. This program also contains editor and solver and it can create PDF. Other features are unlimited undo/redo, save position to file, high scores, hint, show candidates.