One of my classmates started a project LiveClimate.org that lets you offset your CO2 by donating money to specific projects that “remove CO2 from the atmosphere for you”.
I am very much impressed with the site’s design, flow and integration. They use a shopping cart to add your donations to specific projects, a clever use of shopping cart and have an amazing photos widget. As a great example of user interface design, their footprint calculator links directly to the “Shopping Cart”. While they ask you for a donation of $21 per tonne of CO2, they are very accommodative, instead of typing your carbon footprint, you can type in the amount you want to donate.
There is no excuse these days to have a mediocre web design. I really would hire the people who did their website design.