Most free sudoku sites feel stuck in 2010 or buried in ads. Sudoku247Online is a clean, fast, no-signup web app with the classics plus Killer Sudoku, two new harder tiers (Master at 18-19 givens, Evil at the proven 17-given floor), and unlimited printable puzzles in multiple layouts. Features include a graduated hint system (Reveal Cell, Reveal Box, or free Show Candidates), full pencil-mark support with one-tap auto-fill, and a built-in sudoku strategy blog. Works on any device.
