Gerardo64 at the Google Earth BBS has created a file containing global overlays of several datasets available from the NOAA Environmental Visualization Laboratory. The data used to create the maps is collected by satellites and is updated every day.
The following layers are included.
Land Surface Temperature Sea Surface Temperature Snow [...]
The NASA Earth Observations website has dozens of layers of global scientific data formatted for viewing with Google Earth.
The data is grouped into Ocean, Atmosphere, Energy, Land and Life categories. The layers are too numerous to list, but they include:
Sea Surface Temperature Chlorophyll Concentration (MODIS) Snow Cover & Sea Ice Extent [...]
The Cooperative Institute of Meteorological Satellite Studies at the University of Wisconsin-Madison Space Science and Engineering Center (CIMSS-SSEC) has created a huge set of weather related overlays that can be viewed in Google Earth.
Their program is focused on tropical cyclones, but they also have a large amount of satellite and other data [...]
The group over at Natural Earth have put together a great collection of public domain map data and made it freely available online. There are numerous global datasets provided at several different scales. Bad news is most of the data is provided in SHP file format, which does not import directly into the free version [...]