Contents of my boot depend highly on where I'm going and what sort of driving I'm doing and which car I'm in.... for the Spitfire:
Pootling around "locally" - spare tyre, jack, wheel brace, screw drivers, points, length of fuel hose, a few clips, bulbs, jump leads, tow rope and some cable ties. I also have a few lengths of wire with bullets or spade terminals on one end and bare wire on the other incase I need to hotwire lights/ignition etc.
If I am (picking an example purely at random) trying to drive all the way round Britain in 48 hours I'd expand the above to include front wheel bearings pre-packed with grease in a bag, big hammer, ratchet and socket set, crow bar. A decent amount of water (5l), gearbox oil (2l) and engine oil (2l). I also have a small "kit" (spanner, short hose with fittings) that can be used to quickly bridge out the oil cooler.
For adventures further afield (hooning across Europe for example) I add parts that I wouldn't necessarily be able to fit myself at the roadside but wouldn't want to wait a week for them to be shipped from the uk - a drive shaft with bearings etc ready to fit and a gasket set (taped to the inside of the boot lid)