There's this thing happening today - it's called the Blog Action Day, in which bloggers around the world are being asked to post something about the environment. It's a cute idea, and I've been meaning to mention Blackle for a while now. Lots (and lots) of people have Google as their browser's default home page. Google's pages are on a white background. White is the least energy efficient color to display (all colours on) for CRT (the big deep monitors, not the more recent flat ones).
Blackle is Google on a black background. So when you search using Blackle (and assuming you have a CRT monitor), you're saving a teensy weensy bit of electricity. All these teensy weensy bits could add up to something more significant.
Of course if you're using a newer flat (LCD) screen, its back light is always on, so this point is moot.