Blackle is the energy saving search site.
Based on Google, the customised engine uses black as its primary colour scheme, which is apparently more energy effective.
According to their website, they have already saved approximately 96,728 watt hours of electricity.

This is all based on the theory that a black background use less light to output the a bright colour (ie. white) and thus less power, which is a fact, in CRT monitors.
However, on LCD screens it will have little or no effect as the backlight is constantly on and to display black pixels, the monitor merely blocks the backlight, rather than switching it off.

Every little helps though…