From Jens Klausen:
Happy Smile Sudoku should quickly solve all sudokus and can also create new sudokus with or without a symmetry. A symmetry can be chosen among the 4 symmetries most often found in newspapers. It is possible to fix the positions of the clues in a sudoku puzzle that the program creates in advance. The program gives a measure of the difficulty of the sudokus it solves and creates. It is possible in advance to determine the difficulty of a sudoku the program generates. The user can chose the colors of many things that the program displays. One can toggle between 4 different modes of showing hints. One can undo and redo most actions to ease correction of mistakes or to review and learn. In the free version 50 sudoku numbers are available, but each sudoku number can be used to create an enormous amount of different sudokus. In the registered version 100.000 billion sudoku numbers are available, making it easier to create different sudokus.