I don't know where I found this, but it is pretty cool. You make online jigsaw puzzles. No signup required and it is fun. Give the two below a try and let me know what you think. I think I could make a few of these and use them as some sort of learning object in my class. It would at least be a fun way to kill the last five minutes of the day on those days.
If you want to try one that is really hard, try this one: