http://www.stumbleupon.com/earthday/

From the page: “Every day millions of StumbleUpon users discover great web sites and give them a “Thumbs-up”. Starting this Earth Day, we want to harness the power of our community and the “Thumbs-up” to plant trees for the National Forest Foundation.

Simply use the StumbleUpon toolbar to thumbs-up this page and, from now through May 2, with every thumbs-up StumbleUpon will plant a tree with help from the National Forest Foundation*. Your thumbs-up means more trees for America’s National Forests and more visibility for this cause. Give a “Thumbs-Up for Trees” and spread the word!”