Blackle is a front end to Google that works exactly like Google, but with one difference: Its home page and its results pages are black. Why? To save the planet. As it says on the about page, "Blackle saves energy because the screen is predominantly black." The page quotes a 2002 report that maintains, "a given monitor requires more power to display a white (or light) screen than a black (or dark) screen."
That makes intuitive sense when you're talking about CRTs, but not LCD monitors or laptops, which power their backlight consistently regardless of the image the LCD in front of them is displaying.
Blackle creator Toby Heap pointed me back to the report (PDF file), which, on page 19, shows a tiny difference between power consumed when an LCD monitor is displaying a white versus a black screen. But his main point is this: "Regardless, the idea of Blackle is for people to be reminded about saving energy every time they search the Web."
If you really want to save power when on the Web, here's my advice. Turn down the brightness of your screen. That's going to make a bigger difference than using Blackle or not. Better yet, don't surf the Web using a PC with an energy-sucking $500 high-end graphics card. Why do you think some gaming PCs now come with 1 kilowatt power supplies?
Blackle does look cool, though. Very ninja.