Global Change Master Directory

"The mission of NASA's Global Change Master Directory (GCMD) is to assist the scientific community in the discovery of and linkage to Earth science data, as well as to provide data holders a means to advertise their data to the Earth science community."

See their Master Directory http://gcmd.gsfc.nasa.gov/Aboutus/sitemap.html 

See also their main Q&A page: http://gcmd.gsfc.nasa.gov/Resources/Learning/faq.html 

Source: GCMD website