The number of green web hosting options has expanded quite a bit in recent months. Which one should you choose? Well, right now it appears that the primary discriminator is the method by which they power their operation. One group buys Renewable Energy Certificates; these insure that the power they use is generated in an ecofriendly manner. This is typically wind or solar, but it could also be biogas or geothermal as well. Dreamhost is in this category. The second group actually generates their own power directly from renewable energy; AISO, for example, is in this category; they are 100 percent powered by solar that they generate themselves. Treehugger Mike lists a bunch more in this group.
A few offer other deal sweeteners such as carbon offsetting, discounts to non profits, letting their employees telecommute, etc. Some, such as Acorn and ecoSky tout only free software for their hosts. And Green web host combines a bunch of them; they will plant a tree on your behalf, have solar powered offices, and are also paperless. Finally, let's not forget about the money; prices for green hosting services vary widely.
So, which one? It's a simple answer - any one. Because any of these guys are better than any web hoster that hasn't announced a program. Select a price point and options, and make the switch. :: Treehugger