The World Wildlife Fund (WWF) has launched a new program called NEWtrees, through which you get to buy a tree for only $5.50, which is then planted in a rainforest in Sebangau National Forest in Central Kalimantan, Indonesia and you are given the possibility to watch it grow on Google Earth (they send you a Google Earth KML file with the coordinates of your tree).
As Google Earth is updated with higher resolution satellite and aerial images, you should be able to keep an eye on your tree as it grows.
To see already planted trees and buy one of your own, you can check out: mybabytree.org.
NEWtrees is a joint initiative by WWF Indonesia, Nokia and Equinox Publishing.
I personally think this is a great way of encouraging people to actively support replanting projects for forests in Indonesia and across the world; hopefully more such projects will be launched to cover more areas and forests.
You can find a number of other good causes where Google Earth is coming in very handy here: Google Earth Outreach