Pair of jumper cables
If I get a flat battery.
Oil
If oil warning light comes on.
Water
If coolant needs topping up to stop overheating. Water also has a multitude of uses. The empty bottle can also be used for mobile urination when you can't afford the time to pull over for the toilet.
De-icer/scraper
When it gets really cold in Feb/March, I've often come out of work or a pub etc with a frozen windscreen.
Condoms.
Used one so far, and I considered myself a genius for having them there.