Man this thing is addictive, and more fun than crossword puzzles.
Click here if you don't know what it is
Basically it's a number game played on a 9x9 grid. You are to fill the spaces with numbers from 1-9, and cannot repeat any number in any column or row. Within the grid are 9 3x3 blocks, and each much contain numbers from 1-9 without repeating. Try it if you haven't.