Founded in 2003, the motto of Carbonfund.org is "Reduce what you can. Offset what you can't." Their mission is to "encourage everyone to continually strive to reduce their carbon footprint through sensible energy reductions combined with cost-effective carbon offsets to eliminate their overall carbon footprint."

If you want to shrink that number, read recommendations on the website of how to reduce your carbon footprint. For example, keep your car tires properly inflated, upgrade your lighting and appliances and buy local. There's also an entire section on tips for your wedding.
The other half of the equation is offsetting your footprint. Carbonfund.org supports three types of offset projects: energy efficiency, forestry and renewable energy. For example, providing technology at truck stops so drivers can power the interior of their trucks without idling their engines; reforesting 25,000 acres of abandoned cattle pastures in Panama; and promoting wind turbines in China which generate energy, reduce carbon dioxide and provide local employment opportunities. You can read about those projects and many more at the links above.
So how does all this work? As an individual you can calculate your carbon footprint using preset options or by inputing specific information about your home, car and flights. Use the resulting dollar amount and buy some offsets to support the various projects. You can also use your money to plant trees. At a cost of $1.00 per tree, it's an affordable way to improve air quality, preserve biodiversity, control flooding and provide habitat for wildlife. Businesses can participate too.
If calculating your footprint and purchasing carbon offsets is too much effort, you can still support Carbonfund.org by sending in an old school donation, liking their Facebook page and following them on Twitter @Carbonfundorg.