There are a couple of good threads on tool kits in the forums.
In one of the threads, someone posted a really good idea: Have a short range tool kit, and a supplemental larger kit for longer touring.
Couple of points to add to the posts below:
1. Try to get/assemble an OEM tool kit. Especially the "Philips" drivers, as they're not really Philips but JIS, and they don't torque out like regular Philips drivers. It's also nice to have the spark plug wrench.
2. Pack extra fuses
3. If you're running points, it's probably not a bad idea to include a small continuity light (I made a small one out of an LED and some alligator clips on the end of two wires), a feeler gauge, and a points file.
I try to always have a flashlight and Leatherman multi-tool on me when I ride.
I have an F, so I have a little extra room in the storage compartment. I like to put an adjustable wrench in there, a rag, spare fuel filter, and a pair of nitrile gloves.