Mindat.org - A very nice searchable database with a good deal of information on mineral
species, localities, etc.
Galleries.com - Another good link for information on composition, localities, and
other information on mineral species.
Well-Arranged Molecules - A good, reputable dealer of fine mineral specimens at a
reasonable price. He generally carries more higher-end specimens than I do and he
has very good taste. If you do buy from him, let him know that Paul sent you.