I'm totally with Aaron on this, including the specific tool: the Park MTB3 Rescue. As others have said, if you'll be waaaay out in the middle of nowhere you need more. But, for just about anywhere in the U.S., this tool can keep you going unless you get really, really unlucky.

I also carry a fiber spoke, a couple extra 5 mm bolts, a Leatherman and some zip ties.

Finally, I have a good bike dedicated for touring that I keep ready to go, and it is set up with reasonably easy to maintain components.