I came across this light at the local Home Depot:
GE Motion-Sensing LED Light You can also order them online and have them shipped to you if you don't have a local Home Depot.
Even though the reviews weren't stellar, I decided to pick one up and give it a try. For $13 it isn't bad! You can either turn on the light manually or utilize the motion sensor. In the motion sensor mode, the light has a high and a low setting. Probably my only gripe about the light is that the first motion it senses turns it on in the bright mode. The second motion switches it to low mode and the third motion turns it back off again. So, you have to be a little careful, once it's activated, to avoid the sensor. That's not too hard as the sensor doesn't pick up a wide range of motion.
The light has a nice, low profile so it doesn't get in the way of removing things from the top shelf of my safe. I tried other lights previously and this was a problem.
The light is battery powered (three AA batteries) and utilizes an LED light which provides a LOT of light for such a small light and should be fairly conservative on power consumption. I tossed in some Sam's Club brand batteries. We'll see how long they last.
Finally, perhaps the best thing of all ... the light comes with a magnet so installation was literally about one second. I just used the magnet to attach the light to the top inside my safe. The magnet isn't super-strong, though, so the fabric at the top of my safe reduces the magnetism enough that bumping the light could knock it loose.
All in all, if you don't already have a light in your gun safe. This is a really cheap and easy way to put one in.