Forgive me if I'm repeating anything that anyone else already mentioned, but I went through this thread a little faster than I might have.
come-along or winch, flares (keep them in freezer-size Ziplocs), tire chains (for getting out of mud), axe (you never know when you might come across someone trapped in debris and in need of urgent medical help), pipe wrench (for turning off gas lines), wire cutters, 12v plug-in spotlight, flashlight, tp, water, non-perishable food, hygrometer, blankets and pillows, window cleaner and paper towels, a bag of quarters for toll booths, tire pressure gauge, spare fuses, hand cleaner, trash bags, insect repellant
You will find every one of those items, as well as pretty much everything else listed by everyone else, in my little sports coupe while I chase.
Don't forget to install new windshield wipers, and get new tires if you don't have some really good rubber already!