Mineralogy Database (http://www.webmineral.com/)
"This mineral database contains 4,700+ individual mineral species descriptions with links and a comprehensive image library."
American Mineralogist Crystal Structure Database (http://rruff.geo.arizona.edu/AMS/amcsd.php)
Includes crystal structures published in the American Mineralogist, The Canadian Mineralogist, and the European Journal of Mineralogy.
Mineral Gallery (http://mineral.galleries.com/)
Includes physical characteristics, classes, groups, photos of specimens, uses, and common associations.
Mineral Spectrosocpy Server (http://minerals.gps.caltech.edu/)
This server from the California Institute of Technology, "is primarily dedicated to providing information about color in minerals and access to data on Mineral Absorption Spectra in the visible and infrared regions of the spectrum and Raman spectra of minerals."