Marble is a Virtual Globe and World Atlas that you can use to learn more about Earth: You can pan and zoom around and you can look up places and roads. A mouse click on a place label will provide the respective Wikipedia article. You can also measure distances between locations or watch the current cloud cover.
Marble offers several thematic maps: a classroom-style topographic map, a satellite view, street map, earth at night and temperature and precipitation maps. All maps include a custom map key. For educational purposes you can also change date and time and watch how the starry sky and the twilight zone on the map change.
Contrary to other virtual globes Marble also features multiple projections: Choose between a Flat Map (“Plate carré”), Mercator or the Globe.
Marble pulls much of its data from internet servers but can also be used offline. It provides a plugin architecture and can easily be extended for lots of uses. It supports several 3rd party data sources like OpenStreetMap, Wikipedia, KML, GPX etc.
Note that Marble lacks information about some countries (and also check this free Wikipedia Sidebar Gadget with multilanguage support)