Google Earth Map Overlays road maps, terrain relief and contour maps
Here is a Google Earth KML network link that displays a big variety of tile based online maps (road maps, terrain and contour maps, alternative satellite image sets, etc) in the area you are currently viewing.
This allows you to use your favourite map in 3D together with other Google Earth layers to plan a trip, perform geospatial analyses, or simply have better map sources where the data provided by Google is not as good.
Currently, 51 different map sources are supported. Most of these maps are based on OpenStreetMap, but there also layers for Google Maps, Bing Maps and an extensive list of regional map providers. You can switch between maps by clicking on them in the side bar.
Whenever you change the view in Google Earth, the image overlays are automatically updated. Zoom levels for map tiles are calculated independently for different regions in the Google Earth screen, based on distance to the virtual camera and on latitude. Consequently, maps are updated very quickly and the number of map tiles loaded per update is more or less constant, even if the view in Google Earth is tilted.