According to a recent study, it uses less power for a computer's monitor to display a anything (including a Web page) with a black background rather than a white background.
As a result, Google introduced Blackle, a black background based version of Google. It's the exact same thing as regular Google, except with the black background:
The Wall Street Journal did their own study and acknowledges savings, though not as much as the original study claims.
Does it really save power? Even if it saves a tiny bit, I will probably use the black Google page at home, if for no other reason than it looks pretty cool to me.
Link to story