ArcGIS Explorer along with Google Earth. While I've written about Google Earth many times I've only mentioned ArcGIS once even though it is an excellent alternative to Google Earth.
ArcGIS Explorer Online allows users to create mash-ups of geographic data on a map. Users can add data layers to the whole map or to a section of the map. A dozen map base layers are available to build upon. To add data users can select the ArcGIS data base or upload their own data files. As you create in ArcGIS Explorer Online you can switch between "mapping" mode and "presentation" mode. Presentation mode allows you to create map-based slides like you can see in this interactive map of The Odyssey.
ArcGIS does offer free accounts to students. The free accounts are "public" accounts and have some limitations on the amount of information you can map.
Applications for Education
ArcGIS Explorer Online could be a good tool for geography teachers whose students cannot access Google Earth. And for some students ArcGIS Explorer Online might be easier to navigate than Google Earth. In either case ArcGIS Explorer Online provides a good tool that will get students analyzing data in a geographic context quickly.