I probably carry too much but I think Murphy's law applies something like "you will need the one thing you didn't bring, so the more you bring the less your chance of needing something" I have all this:
1. Better jack.
2. A matching spare tire as a lot of vehicles come with compact ones and even when they don't people only buy 4 new tires when it comes time.
3. Better lug wrench. I have a piece of pipe that will slide over the stock one and a 1/2" breaker bar and socket that fit as well.
4. Spare belt. Some engines you can swap with your hands, others need a ratchet or breaker bar or wrench to pull the tensioner pulley.
5. Spare bulbs and fuses and any tools needed to change them. "No need for a ticker officer, I'll fix that right now"
6. When you change your spark plugs and wires, radiator cap, thermostat and radiator hoses as preventative maintenance the old ones still can be used as spares, they can be stuffed in corners under the hood and held in place with plastic tie wraps.
7. Plastic tie wraps, bungie cords, rope, etc. Never know when you might need to strap the bumper on <img src="/images/graemlins/wink.gif" alt="" /> a roll of wire if useful to hold hot exhaust parts on that you drug off when you took that wrong turn down an old road.
8. Small Air compressor and tire patch kit.
9. Flashlights and batteries
10. Change of clothes, boots, jacket/coat and gloves for when you have to change a tire in a rainstorm
11. tow chain and tire chains
12. tarp
13. small blanket
14. tool kit. I found what is called a "wrench roll" a piece of heavy fabric with a bunch of long thin pockets you slide wrenches in and roll it up into small package, roller it up and tucked it under a seat.
15. first aid kit.
16. call phone charger and clips to connect it direct to the car battery. Only non American car I ever owned burned every fuse the week after I bought it and I had an old car phone which didn't have its own battery so I couldn't even call for a tow.
17. jumper cables
18. secondary battery and isolator to keep it charged. Nice for long trips to plug in a small 12v cooler and have it keep running without worrying about running the battery dead. Can jump start myself if I ever need to also.
I have all this tucked away in various spaces of a small truck (s10) and I only have used a small corner of the bed where I have a milk crate sized box for some of the bigger stuff, everything else fits under the hood or seats without taking away from my normal space. Everything is organized and inventoried. I also have a couple extra gauges (voltage for the second battery and trans temp sensor) for better monitoring, a couple extra 12v plugs in the doors so I can plug in things like the compressor easier, a locking rear end for more traction and an cap on the back to keep my cargo dry. Sounds like a lot of $ also but I've bought a little at a time over several years and over half has transferred from an old truck to my new one and them I've added more to it so I haven't had to spend a lot at a time.