If you don't know what to get for your green sister-in-law, or, you do know what to get that Exxon lawyer-in-law and you'd rather piss him/her off (i.e., the real spirit of X-mas), retire some SO2 in their name!
With a small donation, the Acid Rain Retirement Fund will do it for you:
The Acid Rain Retirement Fund was founded to prevent air pollution by buying it right out of the sky. The U.S. Environmental Protection Agency issues pollution allowances or permits that enable companies to emit sulfur dioxide (SO2). These permits are auctioned off to the public and a permit for one ton of sulphur dioxide was recently about $130 [sic]. The Acid Rain Retirement Fund raises funds and bids alongside polluters for as many pollution allowances as our funds can buy. We then retire that pollution credit permanently.
With each donation you'll receive a handsome certificate, suitable for framing and pissing off that special in-law.