Sudoku69 is a free sudoku site with three modes: classic 9x9, 16x16 and killer. Every puzzle is rated by a human-style solver, so the six levels follow the techniques you actually need, not just the number of givens. New daily puzzles for each mode, printable sheets, pencil marks and hints, light, sepia and dark themes, and 38 languages. Installable as an app, no account needed.