Now this app is cool.  EarthViewer is a FREE iPad app that allows users to explore the earth millions of years ago.  Have your students examine all kinds of topics with great visual graphics. Topics like: the greenhouse effect, oxygenation of the planet, and plate tectonics are all included in this extensive app.  EarthViewer was created by BioInteractive.   Along with the app biointeractive.org has great short films, interactive lessons, and virtual labs to complete.  I can see this app and website being used in a science classroom.