Besides the obvious flat-fixing supplies (including tire-booting material which pretty much eliminates the need to carry a spare tire), I'll echo close to the same as above: an allen wrench of each applicable size (except I don't carry the 8mm for removing my Truvativ crankset), a spoke wrench, and a screwdriver for my cleats. I carried a chain tool for 50,000 miles and never needed it, so I quit carrying it. Small screwdrivers are for derailleur adjustment and brake centering, and those simply do not change at all merely from use, no matter how long.
Otherwise, if you know your bike and maintain it, there's no reason to have to fix anything on the road. It's normal to go many, many thousands of miles without a single repair, either at home or on the road. My current bike has 21,000 miles (relatively new), and two things have broken: one aerobar cracked and I had to kind of hold it together to get home (it might have been good to have some kind of tape to tape it up), and a freehub body went out, although it took a few rides to figure out that that was the problem. If your tour were super long, it might be good to have the stuff to remove the cassette and replace the freehub body, and carry a freehub body too; but I wouldn't unless I were leaving the country. If your wheels are well built for the application, it is even unlikely that you'll need spare spokes except possibly in the event of an accident, although it doesn't hurt to take one spoke of each length your wheels use. Changing a rear one might require removing the cassette. I've thought about taking spare cables, but the truth is that I've never had a cable fail on any bike, ever.
