Carbon offsets are financial contributions targeted at projects that take CO2 out of the atmosphere or prevent more from being added. You can purchase offsets to balance travel-generated CO2, typically one offset per ton of gas emitted. The number of tons is based on how many miles you travel, whether by car or plane, and the amount of fuel used. Carbon offsets fund projects that create renewable energy sources, as well as "carbon sequestration" efforts like planting trees.
Several options are listed below:
Atmosfair is a German non-profit company focusing on offsetting air travel.
NativeEnergy is a US-based for-profit company.