Sudoku of the Day (Astraware)

Sudoku of the Day is a long-running sudoku site and app from Astraware, the studio that also makes Mahjong of the Day and Astraware Crosswords. Every day it sets six new puzzles, the same for every player, with one at each level from Beginner up to Diabolical.

The rules are classic sudoku: fill the 9 by 9 grid so every row, column and 3 by 3 box holds 1 to 9. The site and app add help for learning, with manual or automatic pencil marks, smart hints that give a small nudge first and a bigger one if you tap again, and a highlight that shows every instance of a number. When you finish a daily puzzle you can submit your time and compare it with other players worldwide.

The daily page lists each puzzle with a difficulty score and the techniques it calls for, from Single Candidate up to X-Wings and Forcing Chains. Each puzzle can be printed, with or without pencil marks, played in the browser or checked against its solution, and an archive keeps the recent days. The browser player is a plain one: click the grid to start, fill in pencil marks with a button, and a timer and mistake count run beside the board.

The site also has a set of technique guides for going from beginner to expert. In the app, a Puzzles Plus subscription adds unlimited extra puzzles; the daily six are free.

It suits sudoku players of every level, and anyone who wants to step up the difficulty through the week.