1) Terrapass has a carbon footprint calculator for your travel. You can also buy carbon offsets on this site. Your money goes to organizations that fight global warming, all of which have been verified by an independent third party.
2) The Nature Conservancy calculator tabulates your household's carbon footprint.
3) Here's the U.S. Environmental Protection Agency's calculator.
4) The City of Denver has pioneered a method for calculating an entire city's footprint. You can read about it here.
Diana Budds says:
If you're interested in learning about your diet's carbon footprint, check out this calculator: http://www.sustainlane.com/reviews/eat-low-carbon-diet-calculator/MDI1II41PFY78J73HKFRZ8VVNAQI