If you're a mischief-maker of a certain age, you'll remember the idea of the floppy disk bomb, a specially engineered floppy disk that would supposedly melt inside an enemy's drive. This USB is that same basic concept but evolved. It's designed to electrocute your computer from the inside out.
An article about the bomb-drive—first posted on the Russian website Habrahabr by its designer, and then translated into English on Kukuruku Hub—describes its design and intended effect, fortunately without going into the details of how to make one. The idea, roughly, is to fill a standard-looking USB case with hardware that will pulse a high-voltage charge into a USB port and just fry everything nearby.
Per Kukuruku Hub:
When we connect it up to the USB port, an inverting DC/DC converter runs and charges capacitors to -110V. When the voltage is reached, the DC/DC is switched off. At the same time, the filed transistor opens. It is used to apply the -110V to signal lines of the USB interface. When the voltage on capacitors increases to -7V, the transistor closes and the DC/DC starts. The loop runs till everything possible is broken down.
Who knows how well this would actually work, and at best it seems that it might kill one USB drive. That is still a hell of a prank but not exactly the apocalypse. Either way, folks in the comments are clamoring to buy one on Kickstarter, and I'm suddenly a little more wary to just go around jamming strange thumb drives in my ports.