Lisa Jordan, a geography professor at Florida State, recently pointed out to me that there's a wealth of GIS data out there in raster data formats that can't be viewed in the free Google Earth client. For example, Columbia University's Socioeconomic Data Analysis Center generates raster imagery for a variety of different data trends. Although it's possible get higher-resolution vector data (like the main gCensus app does), these files tend to be faster to render and provide a quick overview of relevant trends.
To fill this gap, I've just put out the gCensus-GT application (http://gecensus.stanford.edu/gcensus-gt), which allows you to visualize these raster formats in the free version of Google Earth. In addition to the GeoTIFF format SEDAC generates, gCensus-GT should be able to render any raster format supported by the open-source GDAL library (www.gdal.org) (but GeoTIFF is the only one I've had test data for). Comments are, as always, welcome.
If you're interested in doing this conversion on your home machine, I've released the core code behind it as a Python module named gdaltokmz; it's available at http://www.python.org/pypi/gdaltokmz. The module has a few dependencies (notably, the GDAL Python bindings and the ImageMagick graphics tools) and is licensed under the GPL.