Well, apart from the usual stuff like jack, spare wheel and wheel-spanner.
Inspired by seeing what a colleague has in his boot a few minutes ago in the parking garage (everything from a medical aid kit, emergency stashes of food, etc )
For me, just a change of clothes for the occasions I need to sleep over somewhere, a torch and an empty 5 litre container. The latter as a result of my first car (1992 Fox) which didn't have a working gauge and which had the habit of running out of juice at the most impossible times! I know its silly that I still drag the container around, but old habits die hard