Control the backlight brightness.
I had some problems with xbacklight in combination with the proprietary nvidia driver, so I wrote a replacement.
makesudo make installsudo make uninstall$ backlight_control
Usage: backlight_control [+|-]<value>
Examples:
backlight_control +10
backlight_control -10
backlight_control 50
backlight_control +n increases brightness by n%
backlight_control -n decreases brightness by n%
backlight_control n sets the brightness to n%