Saw this as I was booking a flight this morning. Orbitz and Carbonfund have partnered to provide travelers with an easy to use widget to offset the carbon generated by their national and international flights:

Looks like the offset option is only available when booking a flight. I suspect that it wouldn’t been too difficult for Orbitz and Carbonfund to figure out a way to provide the service for customers who book hotels, rental cars, and cruises, as well. According to the list of partners, Orbitz is the only major travel site to partner with Carbonfund.