Google Earth lets you fly anywhere on Earth to view satellite imagery, maps, terrain, 3D buildings and even explore galaxies in the Sky. You can explore rich geographical content, save your toured places and share with others. (from Google Earth Page)

ArcGIS Explorer delivers an easy-to-use way to view and share information geographically. Download ArcGIS Explorer and start using it to unlock the power of geography and geographic information. ArcGIS Explorer is free and freely distributable, making it a great way to share your maps with others. (from ESRI site)

World Wind lets you zoom from satellite altitude into any place on Earth. Leveraging Landsat satellite imagery and Shuttle Radar Topography Mission data, World Wind lets you experience Earth terrain in visually rich 3D, just as if you were really there.

Virtually visit any place in the world. Look across the Andes, into the Grand Canyon, over the Alps, or along the African Sahara. (from Nasa site)