This website is connected to National Geographic. It allows students to explore difficult topics such as Ecology and Geology through a game like experience. Students work on "missions" as they learn about our Earth.
I found this site through the RSS feed on Google Reader. The RSS stemmed from TechLearning. This site would probably be most applicable for 4th grade and up if it is to be used independently. I think with scaffolding, working as a whole group, and some frontloading of information, younger students could enjoy the experience as well.