There are many companies that will recycle your old cell phone. They range from ones who will buy the phone from you for a few bucks, to ones that will donate to your chosen charity, and even one who will plant a tree for every cell phone donated.
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Recycle Your Cell Phone, Plant a Tree

Support The Arbor Day Foundation
Support The Arbor Day Foundation when purchasing Visa gift cards from www.GiftCards.com. During Earth Week, one dollar from every consumer gift card ordered will be sent to The Arbor Day Foundation. Show your support today!

Three Websites That Help the Environment (and Your Wallet)
Here are three great websites that allow you to donate your unwanted items (thereby avoiding filling up a landfill) and also lets you get free items that other people no longer want. Check them out...
Oxfam America Unwrapped
Oxfam America works in 26 countries around the world. This catalog contains gift items that symbolically represent our work. The items selected represent project goals from grants disbursed by our seven offices around the world. The purchase of each gift item is a contribution toward Oxfam America's many programs, not a donation to a specific project or goal. Your donation will be used where it is needed the most--to help people living in poverty throughout the world.
Tabasco and Chiapas, victims of Global Warming
If you still believe there's no problem with global warming
Top 15 Charity Search Engines: Fast Search Engines that Slow Global Warming and More, Free
Can your search engine offset your carbon footprint? How about funding breast cancer research, environmental conservation, or pubic schools? Certain search engines donate their advertising earnings to charity, sometimes to the charity of your choice. All you have to do is to know where to search.