Some of these suggestions that may or may not apply depending no the age of your vehicle and if you have to venture off the pavement....

I seem to have changed a lot of tires so:
Portable 12V compressor
Tire repair kit and/or tire snot
A length of aluminum pipe or something similar to enhance the leverage of the crappy short lug wrench that comes with the car.
Throw-away gloves for changing tires, a rag or wet-wipes.

Plus:
Some JB Weld (e.g. to fix a radiator or oil pan)
Some insulated electrical wire
Spare fluids such as motor oil and brake fluid
Spare belts, hose clamps
A tow-strap
A fire-extinguisher