My super-favorite pen light is the Streamlight Stylus. Don’t bother with the Pro with fancy housing — go for the simple penlight version. Since the stylus lights are only slightly larger than a ballpoint pen, they go everywhere; in my backpack, my pencil cup on my desk, next to the bed, in my car. Amazingly useful at moments when one needs to look under your seat on an airplane, etc. It’s a great EMT light for looking in eyes and ears, etc. And believe it or not, I find them very useful on official search-and-rescue missions. I always have one within arm’s reach and keep giving them away to friends (they cost less then $12), so I probably buy 10+ year. They run on hard-to-find AAAA batteries, but I generally lose my stylus lights before I need to change the batteries. However, inside a 9-volt battery are 6 4A batteries if you really need some.