Let there be light!
This headlamp weighs only 1.1 oz., and the battery lasts forever. A few times, we've have to hike out in the dark of night and been glad we had these things. Also useful for breaking camp early before the sun is up.
I always carry a spare battery in my first aid kit. I figure with 3 batteries between us, that should get us out of any sticky situations.
Available from REI for $19.95.