Mineralogy Database contains more than 5,000 web pages of mineral data for more than 4,000 mineral species. Data tables provided include crystallography, X-Ray powder diffraction, chemical composition, physical and optical properties, Dana's New classification, Strunz classification, and alphabetical listings of mineral species. An excellent reference site. Both auction sites are user friendly and have provided me with some really great stuff. This is a commercial site offering an excellent assortment of really fine mineral specimens complete with color photos.
Prices are reasonable and there is an weekly auction of specimens as well.
I have several pieces from here in my collection. The Minerals and Mining Page at Albany Stamp Company contains an excellent selection of worldwide rock and mineral postage stamps, including many new issues. Images are available of some stamps. Rockhounding Arkansas is an excellent site authored by a fellow geologist. The site provides information on Arkansas minerals and collecting localities, the ins and outs of rock and mineral collecting, and much more.