Park Tool BSH-4 Bladed Spoke Holder
The BSH-4 is a good little gadget to have on hand if you're building or truing a wheel with bladed spokes. It keeps the spokes from twisting and therefore losing their aero advantage.
The BSH-4 is notched to fit up four different spoke sizes, including big thick ones from Mavic and Fulcrum for example, and works equally well left or right handed.
Charles P said it right. If you don't keep an aero spoke from rotating while truing, it will eventually break. This is an absolutely necessary tool if you work on your own aero spoked wheels.
Those dang aero wheels! What a pain to true, the flat spokes always twist when you turn the nipples. This bad boy will hold the spokes and prevent the frustrating twisting. It has 4 different width of slots so it will work with different thicknesses of spokes. I use this in my left hand to hold the spoke still while spoke wrenching with my right hand. A little awkward but you'll get the hang of it. Made of plastic so it won't scratch the paint off the spokes (especially the fragile Mavic spoke finishes) and long enough to hold comfortably and give good leverage.