Server farms are known to be very energy consuming. A common server uses up hundreds of watts of electricity, and runs on a 24/7 schedule.
Luckily, some companies have decided to minimize their impact on our planet by utilizing renewable energy sources as their main power supplies.
A webhosting company powering its servers by Solar or Wind energy can spare the world from quite a bit of unnecessary CO2 emissions. It may not be an astronomical contribution, but it's another step in the right direction.
I'm going to recommend you a webhosting company that does exactly that. http://www.thinkhost.com/ . Their hosting package is a very handy 100GB of space with 1TB of bandwidth, all provided for about $8. If you're a web developer, or even the owner of this website - please consider this service. Companies that are environmentally aware are rare, and they need all the support they can get.
(You sure can remove my affiliate ID from the URL if you like, but I would appreciate it if you didn't :)