A New Puzzle Challenges Math Skills - New York Times article by Will Shortz about the development of KenKen and how it is played. Wikipedia:KenKen - Online encyclopedia article about the game. MathDoku - Many free puzzles playable online, in three sizes: 4x4, 6x6, and 8x8, in three difficulty levels. Also has "no-op" puzzles (where the operator clue is not given). Online Calcudoku Puzzles - Daily Calcudoku (sizes 4x4 to 12x12) puzzles which can be solved for points. NekNek puzzles - Sudoku + Math - Site with 1200 Calcudoku puzzles, sizes 4x4 to 9x9, solvable online. CalcuDoku - Site with puzzles playable online, tutorial, tips, etc. Math Games, KenKen - Puzzles playable online, sizes 3x3 to 9x9, as well as tutorials and tips.