The Economist has an interesting article comparing the cost per ton of various forms of CO2 abatement. A negative cost per ton means that the abatement saves money and reduces emissions at the same time: better insulation, more efficient light bulbs, and fuel efficient cars all fall into this category. A positive cost per ton means that the abatement reduces emissions, but also costs money: alternative energy sources and planting trees, for instance.
Unfortunately, the positive cost methods get most of the media attention, government investment, and so forth, even as they fight an uphill battle because they are, in fact, more expensive. The negative cost methods are, for the most part, low-tech and boring. Who cares about light bulbs? Let's build wind farms! Except the light bulbs will pay for themselves many times over before the wind farm gets through its environmental impact review.
(Link by way of DD's Eco Notes.)