A lot of what to carry varies with who is in the car, how far from "help" you plan to go and the local conditions. I have always had the following in my car (starting at age 16 when I got my first car). This is probably overkill for many folks but growing up with a father and grandfather who were very good mechanics has had an influence. We lived out of town so a drive anywhere took some time and Midwest weather changes a lot over the year and even during the day.

The list roughly follows an old saying from horse cavalry days, "the horse, the saddle, the man."

Fuses - appropriate for the car, not just a random mix
Good jumper cables - heavy duty and long enough to reach across the car
Tow chain / strap - longer/ stronger than you think you need
Quart of Oil
Quart of Transmission fluid
(Add a gallon of washer fluid/deicer in winter)
Fix-a-Flat (or equiv.)
12V Air pump
tire gauge
tire patch kit (small)
Duct Tape / Muffler Tape
Electricians tape
Small tool kit - at least a couple of screwdrivers and an adjustable wrench/pliers
Locking Pliers
Forceps
Better breaker bar for changing tires - most don't provide enough leverage
Better jack for the car (hydraulic bottle jack usually)

Two signaling devices (currently reflective triangles)
Two flashlights - one larger with kit, one smaller (AA?) in glove box
Several Mechanics Rags / Paper Towels
Leather gloves (all the time, add heavier gloves in fall)
Ice scrapers / snow brushes (fall to spring)
Sand/cat litter for weight/traction (fall to spring)
Something to cut with (knife or scissors)
Seatbelt cutter (in glove box)
Shovel (small)

Blanket/Fleece
First aid kit
Sunscreen
Benadryl fast melt strips
Pain killers (aspirin and tylenol)
car chargers for my electronics (cell phone, gps, iPod...)
At least $2 in change (quarters, dimes, nickels)
a $20 bill

Various consumables (water, snacks etc.) depending on season/climate.

The above can get pretty expensive, especially the stuff from the top of the list. I have always aimed for quality tools and would rather have a few good, simple tools than a complete set of cheap (worthless) wrenches. If you don't know how/when to use the tools, don't bother with them - get AAA and a good cell phone.

- Eric