It's a carbon offset program for websites.
For what it's worth, my company is carbon-neutral, and primarily uses "green" power sources (wind, and nuclear). I do it for two reasons:
1) Promotion. For my business, it's very cheap marketing, and a generator of publicity. For next to nothing, I get a few links, and can differentiate myself from competitors on a dimension that is cared about by some.
2) Because until it's proven that CO2 emissions aren't an issue, I prefer to work on the assumption that they are a problem. After all, if I'm wrong, all I've done is inadvertently invest in energy independence which also strikes me as a good thing :-)