The YC startup CO2Stats charges a fee to analyze the energy footprint of your site and purchase an equivalent amount of wind/solar energy, offsetting your usage. You can then advertise your carbon neutrality with a "green certified site" logo like the one on the bottom of this page.
It is a beginning for sure. I was/am actually looking for applications with which consumers can calculate their personal footprint. A consumer enters his stuff + habits: palm pre, macbook, etc. other data is gathered through scrapping or from other providers (e.g. dopplr). There is a calculation...and voila a user see's his total footprint visually. Based on this information plus additional information a consumer can -- if he desires to -- change his products/habits.