"Carbon offsetting? It's just a joke. To pay money to plant trees, to think you're offsetting the carbon? You're probably making matters worse. You're far better off giving to the charity Cool Earth."
Professor James Lovelock, environmental scientist, author of Gaia
Cool Earth is a charity based in England that buys rainforest. This is pretty standard stuff, but Cool Earth actually partners with local organizations to protect the forest, which means that there is actually someone watching out to make sure that nothing happens to the forest you paid for. It's a pretty new charity, so it's hard to find information on it except for at their home page. I would never have looked at it twice (or at all, honestly), except that James Lovelock approves of it. That's a pretty big endorsement for any charity.
Cool Earth is trying to protect the environment by keeping large amounts of carbon locked up in the rainforests. This is good science. The rain forest traps a lot of carbon dioxide, and when you clear and burn the rainforest all this carbon is released into the atmosphere. Burning an acre means releasing 260 tonnes of carbon dioxide into the atmosphere. Saving the rainforest doesn't do a whole lot to reduce the amount of carbon that we are putting into the atmosphere, but it does prevent many tons of new carbon from being added to the atmosphere.
Cool Earth invests in forest on the edges of major rainforests, the parts of the forest most at risk. It then engages local NGOs to look after the forests. They do allow low level harvesting of plants from the forest it protects, as long as the forest and its ecosystem are allowed to thrive.
It is well worth visiting their site to learn more, at http://www.coolearth.org/. They ask for a basic donation of 15 pounds, which is about American $30 at the moment. Not too bad, for what you get.