Supporting companies that generate alternative sources of electricity is a sound way of offsetting the amount of CO2 in our atmosphere. The energy generated in the US is distributed using a "grid" which distributes the electricity based on demand and availability. The more electricity that is supplied from alternative energy sources such as wind generation, methane digester's, solar collectors, etc., the less electricity is needed from fossil-fueled plants using oil and natural gas. 
Chart from the Native Energy website.
Using less fossil-fuel generated energy results in a "carbon offset" since that much less carbon dioxide (CO2) is getting into the atmosphere, which is a major contributor to global climate change.
Native Energy is helping finance construction of Native American, family farm, and community-based renewable energy projects that feed non-fossil fuel generated electricity into the grid, resulting in carbon offsets. Many individuals as well as companies have chosen to subscribe to Native Energy's efforts by making monthly donations to offset their own carbon footprint.
First you go to their website and calculate your own carbon footprint. Then that number is used to calculate what your monthly donation could be to provide Native Energy the resources they need to offset your carbon-generating activities.

Here is why the process works (from Native Energy)
The Bartel Family wind generator.
A project of Native Energy.
The net result is that for every kilowatt hour generated by a renewable generator, one kilowatt hour less is generated by fossil fuel plants.
Check out Native Energy and discover a very tangible way we can all work toward reducing our demand on the Earth's resources.