Sudoku.com - Sudoku Puzzle

Everybody knows the rules of Sudoku, which is exactly why a good Sudoku app lives or dies on the small details. Sudoku.com keeps the classic 9x9 grid pure and simple, then surrounds it with the tools a paper-and-pencil player would want. There are more than 10,000 puzzles on board, so running out is not really a concern.

Six difficulty levels run from Fast, a gentle warm-up, through Easy, Medium, and Hard, up to Expert and Giant for people who like their grids to fight back. Beginners can learn the ropes at the lower levels without being punished, and the jump in challenge between levels feels steady rather than sudden.

The helpers are well judged. Notes mode works like pencilling in candidates, and the notes update themselves when you place a number. Highlighting shows the row, column, and box around your selected cell and flags duplicate numbers before they ruin a puzzle. Auto-check is there if you want mistakes pointed out as you go, hints rescue you when you are properly stuck, and unlimited undos plus auto-save mean a half-finished grid is always waiting where you left it.

Daily challenges hand out trophies, seasonal events add medals to chase, and the statistics page quietly tracks your best times for each difficulty. It works offline, on phones and tablets alike. If Sudoku is part of your routine — with coffee, on the train, before bed — this is about as complete a version as you will find.