Not really a productivity tool, but apparently some tool that makes working all day long on the screen much more convenient.

“f.lux makes your computer screen look like the room you’re in, all the time. When the sun sets, it makes your computer look like your indoor lights. In the morning, it makes things look like sunlight again”

f.lux adjusts the color temperature of your display to the time of the day. It ranges from 6500 Kelvin during daylight to 3400 Kelvin during the night. Give it a try, downloads are available for Windows XP/Vista, Mac OS X and Linux (glibc6).