Have you ever been in bed computing and your significant other (bed partner) is ready to go to sleep? If you've ever had complaints about the brightness of your monitor at night. Or finally, if you are computing in a completely dark room, sometimes just reducing the brightness of your crisp new laptop screen doesn't cut it. Check out Nocturne (free app from the makers of Quicksilver.)
By launching Nocturne you can completely invert the colors of your screen. What's bright is now dark. And it's surprisingly workable and probably saves a lot of power in the process.
The coolest mode is when you set black to a color, like blue or light-red. It looks like a nightscope for your Mac.
Check it out if you ever compute in the dark. You'll be amazed.