Sudoku

It has nice html5 design. Works on tablets and mobiles, there's no need to download. Great functionality. Pencil marks are available for hard and expert difficulty. Keeps statistics and also shows how your time compares to others. It generates random puzzles from all possible combinations, so there's no chance to get the same puzzle twice. It provides a way to get back to a given puzzle or to challenge a friend. Rules of the game and techniques to solve are also available.