Interesting topic, most normal car owners nowadays would display a face of confusion/disgust if it were suggested that they should carry a tool kit around with them for roadside repairs!
I keep a large ammo box in the back with the standard bottle jack and breaker bars, then the spanner set, socket set (imperial and metric), WD40, duct tape, tow rope, shackes and D-rings, spare fan belts (x2), spare bulbs (3 or 4), spare fuses, screwdrivers, a collection of useful bits of wire and metal, hi-viz waterproofs (getting cold and wet on the side of a busy road is not fun!) and of course the rubber mat in the back comes out for me to lie on under the landy!
Quite an extensive kit and a fair bit of weight but I wouldn't drive anywhere without it!