Spare critical parts and fluids. If you can seal them properly:
2 qt ATF
4 qt (or your vehicle capacity) motor oil
4 qt gasoline
1 pt heet/methanol
mini tool kit
Tire plug kit
12 vdc pump
12 vdc light
12 vdc fan
39-55 gal garbage bags (think containment, poncho, sleeping bag, etc)
large first aid kit
Oh yeah, a bigger bag that you actually leave in your vehicle.
Flashlight or even better a headlamp so you have both hand free if you have to fix your car at night. The Petzl E-Lite is really nice and small if you have small pack, otherwise I would go with the Silva L1 Headlamp. Chemical lightsticks are also good.
Some basic tools.
One or more big container for water (10 liters or more). Some food (freeze dried or MRE:s).
Radio with batteries and preferably with dynamo.
Roadmaps over your regional area. Compass and even better: A GPS.
I keep a GHB(Get Home Bag) as well as a plastic storage trunk like they sell at Wally world for around $17.00. In it on one side is my tool bag and an air compresor for fixing flats. The other half is a milk crate holding 6 Hormel Complete Meals, 3 Ramen Cup od Noodels, 4 large water bottles(around a quart each) paper towels and a box of plastic forks, spoons and knives. Plus a flourecent battery powered work light.
I'm sure some people will say more, some people will say less, but here's a few ideas its all in what YOUR comfortable with.
Pair of old boots / shoes (ever lost a shoe to mud or snow?)
1 full change of clothes
Wool blanket (Don't really like the Mylar Blankets)
Couple Bungie cords
First Aid Kit
Couple MRE's and Retort Water
Water Container of some kind
purification of some kind
fire starting equipment
Firearm or Flair Gun, Subject to legality
Spare fluids / fuses / belt in the trunk of the car
A spare serpentine belt and wrench(socket and breaker bar is better for leverage) to change it with. The weakest parts of your car is the tires, radiator, and belts. They wear also, just like tires but many people dont see the wear out until its too late. Battery post cleaner and wrench for that also is right up there with jumper cables.
Good suggestions already given. I follow them all to some degree. One item I don't believe I saw in any of the above posts that I'd suggest.....leather work gloves for heavy duty work and a light pair of cotton garden gloves for lighter duty work. Might as well throw in a couple pairs of latex gloves and five or so dust masks.