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.
A cool tool is anything useful that is superior to comparable items. If you think this tool is inferior suggest a better one. You are welcome to insult a tool, but comments containing insults to individual people will be deleted. Corrections of fact are always welcomed, if stated politely. Recommendations of better tools are dearly wanted and may be elevated to the front page.