Google Earth is a virtual globe, map and geographic information program that was originally called EarthViewer 3D, and was created by Keyhole, Inc, a company acquired by Google in 2004. It maps the Earth by the superimposition of images obtained from satellite imagery, aerial photography and GIS 3D globe. It is available under three different licenses: Google Earth, a free version with limited functionality; Google Earth Plus (discontinued), which included additional features; and Google Earth Pro ($400 per year), which is intended for commercial use.
Link to a quickstart guide for using Google Earth.
Read this post about how it was used in a class setting.