Park Tool Spoke Tension Meter TM-1
The key to wheels that are strong and reliable is having spokes that are properly and uniformly tensioned. The TM-1 Tension Meter accurately and reliably measures the absolute tension of each of the spokes in a wheel, as well as the relative tension between all the spokes in a wheel. Easy to use and priced accordingly, the TM-1 works on nearly any bicycle spoke no matter what the diameter, material, or shape. Not just for the experienced mechanic, the TM-1 is for anyone building or truing wheels, diagnosing wheel problems, or assembling new bikes. It's a tool that belongs on every workbench.
The product description says that this tool is for everyone who works on their bikes. I disagree. Get a truing stand first. Learn how to true radially, laterally, and dish. Then invest in one of these. You'll be knowledgeable enough to use it by then and motivated enough to actually do so, since you've already spent all this time learning the nuances of your wheels.