The World Wildlife Fund (WWF) has a program called NEWtrees. You can donate a tree and watch it grow on Google Earth. When you buy a tree for $5.50, it will be planted in a rainforest in Sebangau National Forest in Central Kalimantan, Indonesia.
They not only plant the tree for you but you get to watch it grow. You'll also get a Google Earth KML with the coordinates of your tree. As Google Earth gets update with higher resolution satellite and aerial images, you may be able to literally watch your tree grow. You can see the trees planted and donate a tree at mybabytree.org.
NEWtrees is a joint initiative by WWF Indonesia, Nokia and Equinox Publishing. According to Google Earth: "This replanting project gives people an innovative way to help reforest this protected national park and monitor the growth of the trees through geotags (trees labeled with precise latitude and longitude coordinates) viewable via Google Earth and Yahoo Maps."