I found the coolest site this afternoon: http://www.guiweather.com/
Among the many way cool features are a number of KML files for Google Earth. My favorite is the Level II Radar that can display the latest static radar map or a loop automatically updated every five minutes. To install it, with Google Earth open, all you have to do is click on this link.
Another source of weather data including Nexrad Radar and Airport Weather Observations (METARS) is http://earthnc.com/ They also offer all sorts of boating related overlays if your interests sway more to the nautical. In order to access the features you need to register, but registration is quick, easy and free.
Another site with some interesting KMLs is http://www.weatherstationmaps.info/weather/radar/googleEarth.jsp, which is also a cool weather site in its own right.
Finally, one more site with interesting KMLs is http://wdssii.nssl.noaa.gov/geotiff_new/index.html. I particularly like their regional loops, which seem to have better resolution than some of the other sites'.
I could burn lots more time playing with this as Google returned 288,000 hits from a search string of:
Google Earth Weather with online RADAR overlay
but I should go do something more productive.
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