Flashlights and Extra Batteries
Never underestimate the value of a good flashlight. Every car expert under the sun (err, moon?) recommends packing a reliable flashlight in your emergency kit, in case of a breakdown at night. Plus, it can be hard to see what's going on under a car hood, even in the daytime, without flashlight assistance.
Tip: Always store flashlights and new batteries separately — when pre-loaded in a device, batteries can corrode and will definitely loose their juice over time. Put a couple packs of the right-size batteries next to the flashlight in your kit.