Sudoku, the classic 9x9 number-placement puzzle, in the browser: 720 puzzles in six levels (Easy, Medium, Hard, Expert, Master, Extreme), every one proved to have a single solution and graded by the hardest technique it needs, plus a Daily Challenge with a calendar. Pencil notes, hints that fill a cell and explain the technique (3 per game), a mistakes counter with game over at 3 and a second chance, unlimited undo, timer with pause, per-level statistics, light and dark themes, shareable puzzle links. Original design and MIT-licensed code; not affiliated with Easybrain (Sudoku.com), Microsoft or Nikoli. Free, no sign-in, runs entirely in the browser.

Every public app is built from a security-scanned skill and must pass a clean scan — skill and frontend — before it can be listed. Have a skill of your own? Turn it into an app — or read the step-by-step walkthrough.