In the wifes car I only really have the tire changing gear that came with it, as well as a small emergency kit with reflective triangles, jumper cables and first aid kit. We don't get too far off the beaten path so aren't too far from help.
In my pickup I have a few more things.
-water in the summer
-bug spray
-sun screen
-warm.sleeping bag
-rain gear
-extra engine oil and ATF
-a big socket set with an assortment of 1/4", 3/8", and 1/2" sockets, ratchets and extensions as well as allen keys and bit screwdriver
-1/2" torque wrench
-a small box with assorted pliers, screwdrivers, vice grips, hammer, crescent wrench and pipe wrench
-jumper cables
-zip ties
-duct tape
-wd-40
-first aid kit
-knife
-flashlight
-ratchet straps
-winter and summer gloves
I have a dodge megacab so it really doesn't take up much space. I don't use it to repair my truck aside from changing a flat and boosting myself/others, but I take the truck camping, pull my snowmobiles and just generally end up places where I've had to do minor work on other things (visiting people that don't own any tools etc.) I spent $90 on the socket set 10 years ago and everything else is just random stuff I've accumulated over the years so I have no issues if it gets stolen or goes missing.
On a side note, if you ever have to boost a stranger make sure you connect both batteries, no matter how much they argue. I learned that one the hard way, and will leave people stranded before I let them connect their own battery.