Can you guess what it is?
Perhaps if I plug it in and make it do something...
Yes, it's a USB power meter, proving that it's a wonder that any USB hard disks actually work. For non-techies, USB 2.0 ports are only spec'd to supply 500mA, not the 582mA that this drive sucked down on spinup.
There is actually a reason for getting this beyond the shiny toy aspect - the 32GB of internal storage in a Wii U doesn't last long once one starts really using the eShop, but searching online suggests that just buying a 2TB USB hard disk might not be the best idea as the USB ports on it are actually current-limited (almost no-one does this!). The consensus is that bus-powered USB hard disks may initially seem to work, but become unstable under load as the high power draw (probably through a polyfuse) causes the voltage to sag. Hence the power meter, so I can check if I'll need a Y-cable that I also picked up.
And because it's a shiny toy to add to my pile of occasionally-useful computing gadgets, along with the Bus Pirate and the Logic Sniffer.