Web Sudoku is a web site containing billions of free sudoku puzzles. Its home page is divided into two frames: from the left-hand one you can choose the puzzle level or select a variation of the classic game such as a 6-by-6 jigsaw sudoku or a 16-by-16 puzzle while the right-hand frame contains the puzzle board and 5 buttons. The first button is called the "How am I doing?" button, which allows one to see if errors occured while solving the puzzle, or at the end show the time it took for the puzzle to be solved. This time can be compared against other solvers of the same level. Other buttons with less complicated functionality are Print, Pause and Clear. The fifth button is the "Options" button, clicking on which opens an option screens.The options are: Puzzle Timer (Competitve, Regular or Relaxed - no time counting), Personal Statistics, "How am I doing?" - define how errors are to be shown, and Pencil Marking. Checking Pencil Marking allows one to put more than one digit in a square. What is it good for? If you feel you can no longer find the only cell within a row, a column or a 3-by-3 box a cell for a single digit you can put up to 5 possible values in each cell, and reduce them the following way: suppose for a vector or a square of size 9 a subset of n cells which can only contain n values was found, those values should be eliminated from the other cells of the same vector or square.Users of Mac and Microsoft Windows may download an offline versions following the "Play offline with Web Sudoku Deluxe" link.