I've just read an article about Kenken in Spektrum der Wissenschaft. It's a game quite similar to Sudoku, but there are not necessarily any numbers filled in already.
Instead you have a frame around 2 or more fields and a hint like 20×, 2÷, 3+, 5-, so you know what the product, sum, quotient or difference of the numbers within that frame has to be.
Rules are like for Sudoku - in each row and column all numbers must occur once. Quite a fascinating game.
You can find it either here:
http://www.kenken.com (limited to 7x7 kenkens) or here
http://linuxdingsda.de/~wintix/kenken/index.php (up to 20x20 kenkens).