The Princeton Tec Pulsar is one of those lights that everyone should actually carry on their bunch of keys. It weighs seven grams and is only slightly larger than a coin. The white LED has a light output of 10 lumens. This is enough to find a keyhole or to find back something that you have dropped in the dark. Search for the switch is not an issue. You squeeze in the body of the Pulsar and it lights up. The burn time on a button cell is about 12 hours.
The delivery of the Pulsar includes a battery and keychain ring. Of course you can use this to attach the Pulsar to your bunch of keys, but it also works for a zipper, backpack or other places.