Here's an odd toy from Crayola that could function as a tool, too. The Crayola Total Tools Audio Ruler lays down a line of disappearing ink, and then blurts out exactly how long that line is as soon as you're done drawing it. It's accurate to within a quarter of an inch, but has a major limitation.
It can only measure up to a foot in length, which would probably prove to be a drawback if you wanted to use this as an actual tool. If it could resolve measurements down to at least a 16th of an inch, it might be useful for someone who can't see very well. But wait a minute, blind carpenters? It could happen, it really could. Until then, parents will long for the day when rulers remained silent, minding their own business. [Crayola, via Book of Joe]