The Geodata portal at King's College in London is a fantastic collection of geospatial applications that can be visualized and downloaded through a Google Earth or Google Maps interface. It employs region-based network links, which allow you to zoom into areas of interest to visualize the data. There are links for downloading the raw data in simple formats for use in GIS or remote sensing systems. This is a fantastic resource for the classroom. Another specific resource is Terrascope, a Google Earth implementation of the LANDSAT MSS, TM and ETM+ ortho mosaics for the 1970s, circa 1990 and circa 2000. It is designed to allow rapid comparison of LANDSAT imagery between these periods for any view in Google Earth. The data sets also includes a geowiki so that users can move or add information and comments to existing features as well as add new points to the database or comments on visible imagery overlays. All entries are immediately added to the database and visible to all users. It's a great resource that I plan on sharing with my staff.