http://treecar.co.uk/

"At Treecar we plant a tree for every car you hire - you pay nothing extra! This is the most environmentally friendly car site on the internet, but we believe you shouldn’t have to pay more for the privilege. You can help the environment without damaging your pocket.

Treecar is available for all destinations throughout the world. No matter where you hire a car from, irrespective of the duration, one tree will get planted. Treecar is your worldwide, friendly, car hire company."