Cogs in a machine.
When my sister and I were little we had a Spirograph. Of course, we eventually destroyed it, but out of all the toys we had that's the one I probably remember fondest. For years I've wanted another Spirograph set, but I don't think I've ever seen one in a store. But yesterday I found out about several online versions that run in Java. They're not as good as the real plastic things, but they can produce impressive designs, particularly the third one. [Link via Sputterly Utter.]