Three must-haves I carry in my car trunk: a flashlight, jumper cables, and an air compressor for the tires. All pretty self-explanatory, and it just seems incredible to me to not have them. I once got stuck out late at night on a freezing winter evening (pre-cell phone days) with a dead battery because I forgot to turn off the car headlights. Luckily there was someone there who could jump my car and I was able to get home, no problem. Since then, I've always had jumper cables in my car and I've probably jumped about a dozen cars or so. They're not expensive -- maybe $20 to $30 -- but what they give you in peace of mind is so much better.
I also recommend an air compressor. These things are definitely not expensive -- I bought mine for about $20 at AutoZone. This is one of those items that you pay upfront for, but then really appreciate. Previously, I used to have to either go to the garage to get my car tires inflated or search out a gas station to fill my tires. I don't know about you, but the gas stations I always stopped at were either broken or missing a gauge (I have one of those in my car now too). Plus, you always had to have quarters if they didn't give you free air with purchase of gasoline. Now, with my air compressor, it takes about two minutes to fill up the tires in the comfort of my own garage.
I calculated it costs me about $1.50 to fill up all four tires at the gas station and when gasoline cost $4/gallon, it was about 70 cents to travel 4 miles to the gas station. So it's about $2.20/month to fill up my tires. I've owned the pump for more than a year now, so I've easily made that money back. Plus, it's just *convenient*. I can justify cost all I want, but really, it's just the convenience of knowing I can plug this thing into the cigarette lighter in the safety of my own garage and it's just all around better situation.
So, I definitely recommend just having jumper cables and air compressors for tires. It's an up-front investment, but I think worth every penny. Adding a flashlight and a tire pressure gauge for kicks isn't a bad idea either.