Here's a fun extension from the experimental section of Mozilla's Firefox add-ons site. It's called Puzzle, and once installed lets you turn any image from a page you're on into a sectional puzzle with pieces that can be moved around. Upon completion you have the option to ramp up the difficulty, which goes all the way up to a 15x15 grid. It's also able to resize larger images into smaller, laptop-friendly versions.
To toggle it on you simply right click on an image and select which difficulty you want. It then opens up the image (in puzzle form) in a new tab.
I didn't have any luck getting it to work with Firefox 3.1 beta 2, however it worked fine in the current public build of Firefox 3. As with all experimental add-ons, you'll need to be registered with Mozilla to download it.