Shikaku (四角), which means four corners in Japanese, is a fun puzzle game where you fill a puzzle board with quadrilaterals (squares and rectangles) using the clues on the board. Each quadrilateral must contain only 1 number and have the area given by that number.
It's a fun little game to help students connect area with multiplication (a quadrilateral with area 6 can be 1x6, 2x3, 3x2, or 6x1), discover properties of prime and composite numbers (all prime numbers n can only be drawn with 1xn, or nx1 and no prime number can be drawn as a square), and reasoning (some quadrilaterals will have to be redrawn as you encounter limitations).
Here's a video of me playing this Shikaku puzzle.
You can find more puzzles on the Nikoli Shikaku page.
Note: Shikaku was created by the same folks popularized Sudoku.
(via Breedeen Murray)
UPDATE: Changed "came up with" to "popularized" and added link to wikipedia in comments.