Bank of America is offering the Brighter Planet Visa card. For every dollar you spend, you earn 1 EarthSmart Point which is in turn used to support "a geographically and technologically diverse portfolio of renewable energy projects that help reduce your carbon footprint". They estimate 1000 points earned will fund projects that create 1 ton of carbon offsets.
Bright Planet's current supplier of carbon offsets is NativeEnergy. You can go directly to NativeEnergy and buy a 1 ton offset for $35. Using the card you will have to spend $1000 to accomplish the same thing. Compared it to my Delta SkyMiles Card, that is more bang for the buck...
25,000 SkyMiles = 1 round trip ticket in the continental US = $500
That's $20 in rewards per $1000 spent compared to the $35 reward from the Brighter Planet Card!