Sudoku needs no introduction, but it does need honesty about difficulty, so every puzzle here states the hardest technique you will actually need to finish it. Classic Sudoku is a medium 9x9 with 33 given numbers, solvable on hidden singles alone — no branching, no point where the only move left is a coin flip.
The grid takes keyboard or tap input, keeps your pencil marks separate from your answers, and remembers where you got to. Help tokens will reveal a single cell or a whole row, or check what you have entered so far, if you would rather find a mistake now than at the end.
Classic is the only category for the moment, so this page is short. It prints cleanly on one sheet with the solution on another, if you would rather do it on paper.