Yesterday's really useful item was a spare headlight bulb. I pulled into the parking lot at work and realized my driver's side headlight was out. Not a horrible problem, but since I'd be driving home in the dark I popped in the spare. The last time this happened I had to hit a Walmart on my lunch break.
The nice thing about my car is that you can change the bulbs without tools. The not-nice thing is that you can't do anything of the sort with the taillights. Changing a taillight requires disassembling most of the trunk liner then using at least two different metric wrenches to dismount the tail light assembly. Space is tight enough that you can't get around the two wrench requirement with an adjustable crescent wrench either.