Here's another of the websites I have recently discovered thanks to this forum post on the TES Mathematics forum (http://community.tes.co.uk/forums/t/593117.aspx) - Shikaku puzzles!
A Shikaku puzzle is also known as 'rectangles' apparently and the idea is that you have a square grid like those shown below and there are a number of numbers placed in the grid for you as in the left picture.
The idea is to then create squares or rectangles around the numbers so that the area created is the same as the number within the square/rectangle. All squares in the grid must be used and they can get quite challenging as you increase the size of the square grids you are using.
You can print off puzzles like these from the following website... http://www.puzzle-shikaku.com/
A completed puzzle can be seen below, the left pic is the Shikaku puzzle you would be given and the right pic is the solution to it.
Another great puzzle to give students for extension work/form time activities etc etc