Instead of offsetting your CO2-emissions there is now also the possibility to retire them via "Carbon Retirement":

http://www.carbonretirement.com/

Check out their FAQs and How retirement works

A consumer guide to retail carbon offset providers.   Gute Zusammenfasuung ueber carbon offsets und die "besten" Kompanien.

Stockhol Environmental Institute.  Voluntary carbon Offset Information Portal

Consumers who are concerned about the extent of their environmental impact, but who cannot avoid flying completely, may wish to neutralize their travel emissions by purchasing carbon offsets. Individuals can calculate the amount of carbon they are personally responsible for and then purchase an offset for that amount. The funds the offset company receives are then used to implement and manage projects that avoid, reduce or absorb greenhouse gases through renewable energy, energy efficiency, or forest and other bio-sequestration projects.

Native Enegy

Questions about Renewable Energy Credits and CO₂ offsets

• What are renewable energy credits?
• What are CO₂ offsets?
• Are some CO₂ offsets better than others?
• What's the difference between a REC and a CO₂ Offset?
• Why do some RECs and CO₂ offsets cost more or less than others?
• What is the importance of "additionality"?
• Why are renewable energy credits sold separately from the power?
• How does renewable energy reduce CO₂ pollution?
• How do you estimate how much CO₂ pollution is reduced?
• If I'm buying the credits, who gets the power?
• What if my project isn't built?
• What if my project breaks down?
• Do I still have to buy electricity?
• Are these credits just permissions to pollute?
• What if my landlord pays for my electricity?

Questions about NativeEnergy

• Are we a non-profit?
• How much goes to the projects?
• Is this tax deductible?
• Are we a Native American company?
• Who is Intertribal COUP?
• Who is Clean Air-Cool Planet?
• Do I have to donate my RECs/offsets to Clean Air-Cool Planet?