Here's something interesting our Sustainability Jess told us all about on Monday morning...
When your screen is white, be it an empty word page, or the Google page, your computer consumes 74 watts of energy, and when it's black it consumes only 59 watts. A guy Mark Ontkush wrote this article about the energy saving that would be achieved if Google had a black screen. Taking in account their huge number of page views, according to his calculations, 750 mega watts/hour per year would be saved.
In a response to this article Google created a black version of its search engine, called Blackle (or black google if you still love the google logo), with the exact same functions as the white version, but with lower energy consumption.
It's great that google are so open to ideas like this and went and implemented it off that back of Mark's idea. Why don't you go and change the bookmark in your browser for Google. I just have.