The NASA Jet Propulsion Laboratory at the California Institute of Technology has numerous global overlays available that can be viewed with Google Earth. These overlays are on WMS servers, which can be a little tricky to set up in Google Earth, so I have done the hard part and all you have to do is download the file below the screenshot to access all of them. This following layers are included:
1990 Land Characteristics – The Multi-Resolution Land Characteristics Consortium (MRLC) layer covers the continental United States and shows general land characteristics from 1990. This overlay is available in eight different formats (real color, pseudo-color, red band, etc.)
Landsat7 Global Mosaic - The Landsat7 layer covers the entire Earth and shows a mosaic of imagery obtained from the Landsat7 satellite. I believe this imagery was probably obtained in the early 2000’s. This overlay is available in 15 different formats (real color, pseudo-color, Pan Sharpened, red band, thermal band, etc.)
Blue Marble Next Generation Global Mosaic - The Blue Marble layer covers the entire Earth and shows a mosaic of imagery obtained from the MODIS sensor on the Aqua and Terra satellites. I believe this imagery was probably obtained in the mid 2000’s. There are separate mosaics for each month of the year and also various other options, such as the ability to view the mosaics with or without bathymetry shading.
SRTM Reflectance - The SRTM Reflectance layer covers the entire Earth and shows data collected by the Space Shuttle in 2000. This data was used to generate a highly accurate elevation grid of the Earth. There are separate mosaics for each of the four subswaths and and an average. In addition, a 90 meter SRTM Derived Elevation Model is included.
National Elevation Dataset (NED)- The National Elevation Dataset layer covers the continental United States and shows a grey scale elevation map. 30 meter and 90 meter versions of the overlays are provided.
MODIS Dailey Planet - The Dailey Planet layer covers the entire Earth and shows a continuously updated mosaic of imagery obtained from the MODIS sensor on the Aqua and Terra satellites. I believe the age of this imagery is typically less than 24 hours old with the Terra imagery from the morning and Aqua imagery from the afternoon.
Much more information about each of these layers is available at the OnEarth website
See above descriptions