Planting trees as a way to offset carbon emissions is a bit controversial, since a lot more has to happen to change the plight of Arctic ice than putting trees in the ground. However, reforestation is an absolute must so planting trees is pretty cool, even if it is under the guise of reversing global warming. Going farther than just a search tool bar, Ecocho is a whole search engine that plants trees based on search counts...
Source: TreeHugger