Spoke wrench.
Came as part of my "emergency miracle tool" which I carry in my saddlebag.
One also came with my basic tool set.

p.s. Did you bother to go to the Park Tools site to see what it says about wheel truing? It lists all the tools that you might need to true a wheel. As you read through the instructions, you will see which ones you will actually need...
http://www.parktool.com/repair/readhowto.asp?id=81