Many datacenters are claiming to be "Green" by using alternative energy sources such as solar, wind, etc.

We've been using the Antec Green (80+ Certified) power supplies in many of our servers, there seems to be a noticeable difference in the draw on the "green" servers vs the standard ones. It's not a huge difference, but its better than nothing I suppose. Plus it helps the electricity bill once you have a few hundred green PSU's instead of a few hundred standard PSU's

Although unfortunately it seems a few companies are using it more as a marketing ploy than anything.