Well, my truck and my wife's SUV are vastly different creatures in that regard.
I have a cross-bed box in my truck, and that thing is TOO full.
I have a decent sized poly carry box pretty full of hand tools, mostly from HF or auction scores. I have a standard/metric end wrench set in a roll pouch, a couple of Cresents, some screwdrivers, assorted pliers and some other odds and ends. In addition, I have a small HF 1/4 / 3/8 / 1/2 socket set in it's own thin box. My main purpose for carrying these is not so much for a breakdown, bot just to have them with me at all times. I've used these tools at work, at relatives/friends houses, at church, etc. Also, I go to a lot of auctions, and I sometimes need to dismantle my purchases for hauling home.
In addition to the hand tools, I have a 12V air compressor, which I have used on the truck!, a pair of jumper cables, a large pry bar, a BFH, another hammer which is kind of a rescue tool, a fire extinguisher, warning triangles, a tow chain and axle strap, a shovel, a couple of old large towels (these work for clean up, something to put on the ground if you have to kneel/lay down, or for padding/protection when hauling something), a blue tarp, some plastic bags, duct tape and electrical tape, lan wrap, assorted ratchet and bungee straps, a cargo bar, and a couple of inserts for my receiver hitch. I've probably forgotten a couple of things, but that should be most of it.
My wife on the other hand MAY have a set of jumper cables. But her name is on the AAA card and it's in her purse, along with her i-phone. Plus, she never goes to auctions!