I take one of these with me everywhere - Petzl Zipka LED Headlamp

The brilliant retracting cable on this light makes it more portable and useful than any headlamp I have. Runs for a lonnng time on 3 AAA cells, or you can get a rechargeable battery pack.

Here are just some of the place I keep the Zipka and how I've used it:
- In the glove compartment of my car - hands-free loading, unloading, filling tires, checking oil
- In my briefcase - going through the storage room at work, finding something I dropped on the floor of a hotel room, just in case for a power outage
- In my pannier - my bike light died, and I used it just under my helmet to get home
- In the house - taking garbage and recycling to the bins out back, working on projects after dark, finding a chicken who didn't make it into the coop (try carrying a chicken and a flashlight!)
- In my backpack - good for seeing early morning or late evening trails, answering the call of nature, wrapping it around a tentpole for a tent light

I have one of these for each of my family and each car, and a couple others just in case.

You won't regret the investment.