A nipple driver. It's essentially a screwdriver with a rotating shaft and a bend in the bit that allows you to spin the handle and spin a spoke nipple on to the spoke. These are typically very specialized tools that cost a fair amount ($30 or more!)
I was looking for a less expensive alternative and went to my local hardware store when I found it!
It's a Klein Tools Rapi-Driv©. The hardware store had it for $12.99! Much better than most bike shops want for a similar tool!
I ground down two of the sides down so that it fits in the slots of the nipples and a quick test shows that it works beautifully! It really makes building or truing a wheel very fast and easy!