I usually carry a fuel pump, but only because the car has a lot of miles on it and it can be changed on the side of the road in less than half an hour. Other than that, I just take tools to fix potential problems. If you have tools and your car is built using easy to get parts (i.e. factory) then you'll be able to find a way to get the parts, someone will stop to help. If you don't believe me, stop on the side of the road and try to take a picture of the cars rolling by, someone will stop to make sure you're alright, happed to me several times.
